Add basic support for executeAsyncScript.

This relies on a global webdriverCallback function, which is visible to content.
Obviously that's not a long term solution for a number of reasons, but it allows
us to experiment for now

components/script/dom/webidls/Window.webidl

@@ -56,6 +56,9 @@
 
   //void postMessage(any message, DOMString targetOrigin, optional sequence<Transferable> transfer);
 
+  // Shouldn't be public, but just to make things work for now
+  void webdriverCallback(optional any result);
+
   // also has obsolete members
 };
 Window implements GlobalEventHandlers;