WIP: diff which plumbs through the event URL.

chrome/test/data/extensions/webui/on_message.js

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // out/Debug/browser_tests --gtest_filter=ExtensionWebUITest.OnMessage
 
+if (!chrome || !chrome.test || !chrome.test.onMessage) {
+  console.error('chrome.test.onMessage is unavailable on ' +
+                document.location.href);
+  domAutomationController.send(false);
+  return;
+}
+
 chrome.test.listenOnce(chrome.test.onMessage, function(args) {
   if (args.data != 'hi') {
     console.error('Expected "hi", Actual ' + JSON.stringify(args.data));
     chrome.test.sendMessage('false');
   } else {
     chrome.test.sendMessage('true');
   }
 });
 
 domAutomationController.send(true);