Use DOM's post-connection steps for iframe elements

Make iframe elements use post-connection steps, not insertion steps. Fixes #10828.

source

@@ -33415,13 +33415,10 @@ interface <dfn interface>HTMLIFrameElement</dfn> : <span>HTMLElement</span> {
 
   <hr> <!-- SRC/SRCDOC PROCESSING MODEL -->
 
-  <p>The <code>iframe</code> <span>HTML element insertion steps</span>, given
+  <p>The <code>iframe</code> <span>HTML element post-connection steps</span>, given
   <var>insertedNode</var>, are:</p>
 
   <ol>
-   <li><p>If <var>insertedNode</var>'s <span>shadow-including root</span>'s <span
-   data-x="concept-document-bc">browsing context</span> is null, then return.</p></li>
-
    <li><p><span>Create a new child navigable</span> for <var>insertedNode</var>.</p></li>
 
    <li><p>If <var>insertedNode</var> has a <code data-x="attr-iframe-sandbox">sandbox</code>