Null out LocalDOMWindow::m_frame on navigation.

third_party/WebKit/LayoutTests/fast/dom/Window/resources/dom-access-from-closure-iframe-child.html

 <script>
+    // Capture window.window and window.parent: once this context is navigated, these properties
+    // always return null.
+    var w = window;
+    var p = parent;
     parent.accessFrame = function()
     {
         function normalizeURL(url)
         {
             return url.substring(url.lastIndexOf("LayoutTests"));
         }
 
         try {
-            parent.log("document.URL: " + normalizeURL(document.URL));
-            parent.log("window.document.URL: " + normalizeURL(window.document.URL));
-            parent.log("name: " + name);
-            parent.log("window.name: " + window.name);
+            p.log("document.URL: " + normalizeURL(document.URL));
+            p.log("window.document.URL: " + normalizeURL(w.document.URL));
+            p.log("name: " + name);
+            p.log("window.name: " + w.name);
         } catch (e) {
-            parent.log("An exception was thrown: " + e.message);
+            p.log("An exception was thrown: " + e.message);
         }
     }
 
     window.location = "notify-parent-done.html";
 </script>