Compute "allowed to download" synchronously in navigation

Helps with #1130 by removing a deep-in-the-algorithm-tree usage of source browsing context. Does not close #5597, but fixes the actual problems posed by the current architecture.

source

@@ -84617,6 +84617,10 @@ interface <dfn>Location</dfn> { // but see also <a href="#the-location-interface
    flags</span> given <var>browsingContext</var> and <var>browsingContext</var>'s <span
    data-x="bc-container">container</span>.</p></li>
 
+   <li><p>Let <var>allowedToDownload</var> be the result of running the <span>allowed to
+   download</span> algorithm given the <span>source browsing context</span> and
+   <var>browsingContext</var>.</p></li>
+
    <li><p>Return to whatever algorithm invoked the navigation steps and continue running these steps
    <span>in parallel</span>.</p></li>
 
@@ -84670,8 +84674,8 @@ interface <dfn>Location</dfn> { // but see also <a href="#the-location-interface
        <var>browsingContext</var>, and <span data-x="navigation-params-hh">history
        handling</span> is <var>historyHandling</var>.</p></li>
 
-       <li><p>Run <span>process a navigate response</span> with <var>navigationType</var>, the
-       <span>source browsing context</span>, and <var>navigationParams</var>.</p></li>
+       <li><p>Run <span>process a navigate response</span> with <var>navigationType</var>,
+       <var>allowedToDownload</var>, and <var>navigationParams</var>.</p></li>
       </ol>
      </dd>
 
@@ -84721,8 +84725,8 @@ interface <dfn>Location</dfn> { // but see also <a href="#the-location-interface
        <var>browsingContext</var>, and <span data-x="navigation-params-hh">history
        handling</span> is <var>historyHandling</var>.</p></li>
 
-       <li><p>Run <span>process a navigate response</span> with <var>navigationType</var>, the
-       <span>source browsing context</span>, and <var>navigationParams</var>.</p></li>
+       <li><p>Run <span>process a navigate response</span> with <var>navigationType</var>,
+       <var>allowedToDownload</var>, and <var>navigationParams</var>.</p></li>
       </ol>
 
       <p class="example">So for example a <span data-x="javascript
@@ -84741,7 +84745,7 @@ interface <dfn>Location</dfn> { // but see also <a href="#the-location-interface
      is a <span>fetch scheme</span></dt>
      <dd><p>Run <span>process a navigate fetch</span> given <var>resource</var>, the <span>source
      browsing context</span>, <var>browsingContext</var>, <var>navigationType</var>,
-     <var>sandboxFlags</var>, <var>incumbentNavigationOrigin</var>,
+     <var>sandboxFlags</var>, <var>allowedToDownload</var>, <var>incumbentNavigationOrigin</var>,
      <var>activeDocumentNavigationOrigin</var>, and <var>historyHandling</var>.</p></dd>
 
      <dt>Otherwise, <var>resource</var> is a <span data-x="concept-request">request</span> whose
@@ -84757,10 +84761,10 @@ interface <dfn>Location</dfn> { // but see also <a href="#the-location-interface
   <p>To <dfn export>process a navigate fetch</dfn>, given a <span
   data-x="concept-request">request</span> <var>request</var>, two <span data-x="browsing
   context">browsing contexts</span> <var>sourceBrowsingContext</var> and <var>browsingContext</var>,
-  a string <var>navigationType</var>, a <span>sandboxing flag set</span> <var>sandboxFlags</var>,
-  two <span data-x="origin">origins</span> <var>incumbentNavigationOrigin</var> and
-  <var>activeDocumentNavigationOrigin</var>, and a <span>history handling behavior</span>
-  <var>historyHandling</var>:</p>
+  a string <var>navigationType</var>, a <span>sandboxing flag set</span> <var>sandboxFlags</var>, a
+  boolean <var>allowedToDownload</var>, two <span data-x="origin">origins</span>
+  <var>incumbentNavigationOrigin</var> and <var>activeDocumentNavigationOrigin</var>, and a
+  <span>history handling behavior</span> <var>historyHandling</var>:</p>
 
   <ol>
    <li><p>Let <var>response</var> be null.</p></li>
@@ -84993,7 +84997,7 @@ interface <dfn>Location</dfn> { // but see also <a href="#the-location-interface
    <span>process a navigate fetch</span> with a new <span data-x="concept-request">request</span>
    whose <span data-x="concept-request-url">url</span> is <var>locationURL</var>,
    <var>sourceBrowsingContext</var>, <var>browsingContext</var>, <var>navigationType</var>,
-   <var>sandboxFlags</var>, <var>incumbentNavigationOrigin</var>,
+   <var>allowedToDownload</var>, <var>sandboxFlags</var>, <var>incumbentNavigationOrigin</var>,
    <var>activeDocumentNavigationOrigin</var>, and <var>historyHandling</var>, and return.
 
    <li><p>Otherwise, if <var>locationURL</var> is a <span>URL</span>, run the <span>process a
@@ -85015,12 +85019,12 @@ interface <dfn>Location</dfn> { // but see also <a href="#the-location-interface
    <var>browsingContext</var>, and <span data-x="navigation-params-hh">history handling</span> is
    <var>historyHandling</var>.</p></li>
 
-   <li><p>Run <span>process a navigate response</span> with <var>navigationType</var>, the
-   <span>source browsing context</span>, and <var>navigationParams</var>.</p></li>
+   <li><p>Run <span>process a navigate response</span> with <var>navigationType</var>,
+   <var>allowedToDownload</var>, and <var>navigationParams</var>.</p></li>
   </ol>
 
   <p>To <dfn export>process a navigate response</dfn>, given a string <var>navigationType</var>, a
-  <span>browsing context</span> <var>source</var>, and a <span>navigation params</span>
+  boolean <var>allowedToDownload</var>, and a <span>navigation params</span>
   <var>navigationParams</var>:</p>
 
   <ol>
@@ -85087,8 +85091,7 @@ interface <dfn>Location</dfn> { // but see also <a href="#the-location-interface
     data-x="">attachment</code> disposition type, then:</p>
 
     <ol>
-     <li><p>If the result of running the <span>allowed to download</span> given <var>source</var>
-     and <var>browsingContext</var> is true, then handle <var>response</var> <span>as a
+     <li><p>If <var>allowedToDownload</var> is true, then handle <var>response</var> <span>as a
      download</span>.</p></li>
 
      <li><p>Return.</p></li>