Bug 866748 - isTopLevel shouldn't throw. r=smaug

uriloader/base/nsDocLoader.cpp

@@ -952,17 +952,18 @@ nsDocLoader::GetIsTopLevel(bool *aResult)
   *aResult = false;
 
   nsCOMPtr<nsIDOMWindow> window;
-  nsresult rv = GetDOMWindow(getter_AddRefs(window));
-  NS_ENSURE_SUCCESS(rv, rv);
-
+  GetDOMWindow(getter_AddRefs(window));
+  if (window) {
     nsCOMPtr<nsPIDOMWindow> piwindow = do_QueryInterface(window);
     NS_ENSURE_STATE(piwindow);
 
     nsCOMPtr<nsIDOMWindow> topWindow;
-  rv = piwindow->GetTop(getter_AddRefs(topWindow));
+    nsresult rv = piwindow->GetTop(getter_AddRefs(topWindow));
     NS_ENSURE_SUCCESS(rv, rv);
 
     *aResult = piwindow == topWindow;
+  }
+
   return NS_OK;
 }