Check if there's a browsing context in window.open() steps

Fixes #11797.

Author: zcorpan

source

@@ -95305,6 +95305,9 @@ dictionary <dfn dictionary>WindowPostMessageOptions</dfn> : <span>StructuredSeri
    <li><p>If the <span>event loop</span>'s <span>termination nesting level</span> is nonzero,
    then return null.</p></li>
 
+   <li><p>If the <span>entry global object</span>'s <span data-x="window bc">browsing
+   context</span> is null, then return null.</p></li>
+
    <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>