Stop propagating inertness into nested browsing contexts

Firefox doesn't propagate it. Blink does, but I'm removing it since it
can be a cross-site leak, and blocks fenced frames since it opens a
communication channel. WebKit only propagates it for `.focus()`, but not
for the selection API.

Note that even if the contents in the nested browsing contexts are not
marked as inert, if the browsing context container is inert, they won't
respond to mouse interactions, and they won't be reached by sequential
navigation.

Fixes whatwg#7605.

source

@@ -75037,10 +75037,6 @@ END:VCARD</pre>
   <p class="note">When a node is inert, it generally cannot be focused. Inert nodes that are <span
   data-x="concept-command">commands</span> will also get disabled.</p>
 
-  <p>While a <span>browsing context container</span> is marked as <span>inert</span>, its
-  <span>nested browsing context</span>'s <span>active document</span>, and all nodes in that
-  <code>Document</code>, must be marked as <span>inert</span>.</p>
-
   <p>A <code>Document</code> <var>document</var> is <dfn>blocked by a modal dialog</dfn>
   <var>subject</var> if <var>subject</var> is the topmost <code>dialog</code> element in
   <var>document</var>'s <span>top layer</span>. While <var>document</var> is so blocked, every node