Stop propagating inertness into nested browsing contexts

This can be a cross-site communications channel, and browsers don't want to leave it open.

Note that even if the contents in the nested browsing contexts are not marked as inert, if the browsing context container is inert, the contents won't respond to mouse interactions, and they won't be reached by sequential focus navigation. So they generally behave similarly from the perspective of the user.

Fixes whatwg#7605.

source

@@ -75063,10 +75063,6 @@ 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