Fix window.close() navigable reference

Also clean up the initial steps by using some convenience definitions.

Closes #10418. Note that this does not help with #1866.

source

@@ -89463,20 +89463,11 @@ dictionary <dfn dictionary>WindowPostMessageOptions</dfn> : <span>StructuredSeri
   are:</p>
 
   <ol>
-   <li><p>Let <var>thisTraversable</var> be null.</p></li>
+   <li><p>Let <var>thisTraversable</var> be <span>this</span>'s <span
+   data-x="window-navigable">navigable</span>.</p></li>
 
-   <li><p><span data-x="list iterate">For each</span> <span>top-level traversable</span>
-   <var>traversable</var> of the user agent's <span>top-level traversable set</span>: if
-   <var>traversable</var>'s <span data-x="nav-document">active document</span>'s <span>relevant
-   global object</span> equals <span>this</span>, then set <var>thisTraversable</var> to
-   <var>traversable</var> and <span>break</span>.</p></li>
-
-   <li>
-    <p>If <var>thisTraversable</var> is null, then return.</p>
-
-    <p class="note">In this case the method is being called on a <code>Window</code> that does not
-    correspond to a <span>top-level traversable</span>, and so closing is not allowed.</p>
-   </li>
+   <li><p>If <var>thisTraversable</var> is not a <span>top-level traversable</span>, then
+   return.</p></li>
 
    <li><p>If <var>thisTraversable</var>'s <span>is closing</span> is true, then return.</p></li>
 
@@ -89498,8 +89489,9 @@ dictionary <dfn dictionary>WindowPostMessageOptions</dfn> : <span>StructuredSeri
      <var>browsingContext</var>; and</p></li>
 
      <li id="sandboxClose"><p>the <span data-x="concept-incumbent-global">incumbent global
-     object</span>'s <span>node navigable</span> is <span>allowed by sandboxing to navigate</span>
-     <var>thisTraversable</var>, given <var>sourceSnapshotParams</var></p></li>
+     object</span>'s <span data-x="window-navigable">navigable</span> is <span>allowed by
+     sandboxing to navigate</span> <var>thisTraversable</var>, given
+     <var>sourceSnapshotParams</var></p></li>
     </ul>
 
     <p>then:</p>