[Extensions] Don't allow built-in extensions code to be overridden

extensions/renderer/resources/test_custom_bindings.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.
 
 // test_custom_bindings.js
 // mini-framework for ExtensionApiTest browser tests
 
 var binding = require('binding').Binding.create('test');
 
 var environmentSpecificBindings = require('test_environment_specific_bindings');
@@ skipping 343 matching lines @@
     uncaughtExceptionHandler.setHandler(callback);
   });
 
   apiFunctions.setHandleRequest('getWakeEventPage', function() {
     return natives.GetWakeEventPage();
   });
 
   environmentSpecificBindings.registerHooks(api);
 });
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());