Fix #313: make disowning disown the opener browsing context too

Otherwise window names would still leak across.

source

@@ -77954,17 +77954,20 @@ dictionary <dfn>DragEventInit</dfn> : <span>MouseEventInit</span> {
   the <span>browsing context</span> from which the <span>auxiliary browsing context</span> was
   created.</p>
 
+  <p>When an <span>auxiliary browsing context</span> is to <dfn data-x="disowned its opener">disown
+  its opener</dfn>, its <span>opener browsing context</span> must be set to null. This makes the
+  <span>auxiliary browsing context</span> indistinguishable from a <span>top-level browsing
+  context</span>.
+
 
   <h5>Navigating auxiliary browsing contexts in the DOM</h5>
 
   <p>The <dfn><code data-x="dom-opener">opener</code></dfn> IDL attribute on the <code>Window</code>
-  object, on getting, must return the <code>WindowProxy</code> object of the <span>browsing
-  context</span> from which the current <span>browsing context</span> was created (its <span>opener
-  browsing context</span>), if there is one, if it is still available, and if the current
-  <span>browsing context</span> has not <i data-x="disowned its opener">disowned its opener</i>;
-  otherwise, it must return null. On setting, if the new value is null then the current
-  <span>browsing context</span> must <dfn data-x="disowned its opener">disown its opener</dfn>; if
-  the new value is anything else then the user agent must
+  object, on getting, must return the <code>WindowProxy</code> object of the current <span>browsing
+  context</span>'s <span>opener browsing context</span>, if there is one; otherwise, it must return
+  null. On setting, if the new value is null then the current <span>browsing context</span> must
+  <span data-x="disowned its opener">disown its opener</span>; if the new value is anything else
+  then the user agent must
 
     <!-- dark magic incantation begins -->
     call the [[DefineOwnProperty]] internal method of the <code>Window</code> object, passing the