Change URL parsing sequence for window open steps

Previously, we would only check at the last minute. This would be buggy in the case where we create a new navigable, and then throw an exception. Instead we should be sure to throw before creating a new navigable.

In addition to fixing this somewhat-obviously-wrong bug, this also causes validation to happen before popup blocking or other window-choosing steps. Previously, if trying to open a window to an unparseable URL with the popup blocker active, the window open steps would return null; now, they throw an exception.

Closes #10681.

source

@@ -89800,6 +89800,20 @@ dictionary <dfn dictionary>WindowPostMessageOptions</dfn> : <span>StructuredSeri
    <li><p>Let <var>sourceDocument</var> be the <span>entry global object</span>'s <span
    data-x="concept-document-window">associated <code>Document</code></span>.</p></li>
 
+   <li><p>Let <var>urlRecord</var> be null.</p></li>
+
+   <li>
+    <p>If <var>url</var> is not the empty string, then:</p>
+
+    <ol>
+     <li><p>Set <var>urlRecord</var> to the result of <span>encoding-parsing a URL</span> given
+     <var>url</var>, relative to <var>sourceDocument</var>.</p></li>
+
+     <li><p>If <var>urlRecord</var> is failure, then throw a <span>"<code>SyntaxError</code>"</span>
+     <code>DOMException</code>.</p></li>
+    </ol>
+   </li>
+
    <li><p>If <var>target</var> is the empty string, then set <var>target</var> to "<code
    data-x="">_blank</code>".</p></li>
 
@@ -89871,15 +89885,8 @@ dictionary <dfn dictionary>WindowPostMessageOptions</dfn> : <span>StructuredSeri
      data-x="nav-bc">active browsing context</span> given
      <var>tokenizedFeatures</var>. <ref>CSSOMVIEW</ref></p></li>
 
-     <li><p>Let <var>urlRecord</var> be the <span>URL record</span>
-     <code>about:blank</code>.</p></li>
-
-     <li><p>If <var>url</var> is not the empty string, then set <var>urlRecord</var> to the result
-     of <span>encoding-parsing a URL</span> given <var>url</var>, relative to the <span>entry
-     settings object</span>.</p></li>
-
-     <li><p>If <var>urlRecord</var> is failure, then throw a <span>"<code>SyntaxError</code>"</span>
-     <code>DOMException</code>.</p></li>
+     <li><p>If <var>urlRecord</var> is null, then set <var>urlRecord</var> to a <span>URL
+     record</span> representing <code>about:blank</code>.</p></li>
 
      <li>
       <p>If <var>urlRecord</var> <span>matches <code>about:blank</code></span>, then perform the
@@ -89902,22 +89909,10 @@ dictionary <dfn dictionary>WindowPostMessageOptions</dfn> : <span>StructuredSeri
     <p>Otherwise:</p>
 
     <ol>
-     <li>
-      <p>If <var>url</var> is not the empty string, then:</p>
-
-      <ol>
-       <li><p>Let <var>urlRecord</var> be the result of <span>encoding-parsing a URL</span>
-       <var>url</var>, relative to the <span>entry settings object</span>.</p></li>
-
-       <li><p>If <var>urlRecord</var> is failure, then throw a
-       <span>"<code>SyntaxError</code>"</span> <code>DOMException</code>.</p></li>
-
-       <li><p><span>Navigate</span><!--DONAV window.open()--> <var>targetNavigable</var> to
-       <var>urlRecord</var> using <var>sourceDocument</var>, with <i
-       data-x="navigation-referrer-policy">referrerPolicy</i> set to <var>referrerPolicy</var> and
-       <i><span>exceptionsEnabled</span></i> set to true.</p></li>
-      </ol>
-     </li>
+     <li><!--DONAV window.open()--><p>If <var>urlRecord</var> is not null, then
+     <span>navigate</span> <var>targetNavigable</var> to <var>urlRecord</var> using
+     <var>sourceDocument</var>, with <i data-x="navigation-referrer-policy">referrerPolicy</i> set
+     to <var>referrerPolicy</var> and <i><span>exceptionsEnabled</span></i> set to true.</p></li>
 
      <li><p>If <var>noopener</var> is false, then set <var>targetNavigable</var>'s <span
      data-x="nav-bc">active browsing context</span>'s <span>opener browsing context</span> to