Define "child browsing context" to pierce shadow trees

Part of WICG/webcomponents#145. Although we
don't want to expose such shadow-encapsulated browsing contexts via
indexed or named property access on the Window, we in general do want
to treat them as child browsing contexts. (A subsequent commit will
change named and indexed property access.)

source

@@ -77133,10 +77133,10 @@ dictionary <dfn>DragEventInit</dfn> : <span>MouseEventInit</span> {
    <li><p><var>child</var> is a <span>nested browsing context</span> of a <span>browsing context
    container</span> <var>element</var></p></li>
 
-   <li><p><var>element</var> is <span>in a document</span> <var>document</var></p></li>
+   <li><p><var>element</var> is <span>connected</span></p></li>
 
-   <li><p><var>document</var>'s <span data-x="concept-document-bc">browsing context</span> is
-   <var>parent</var></p></li>
+   <li><p><var>element</var>'s <span>shadow-including root</span>'s <span
+   data-x="concept-document-bc">browsing context</span> is <var>parent</var></p></li>
   </ul>
 
   <p>A <span>browsing context</span> <var>child</var> may have a <dfn>parent browsing context</dfn>.