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

extensions/renderer/resources/event.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.
 
   var exceptionHandler = require('uncaught_exception_handler');
   var eventNatives = requireNative('event_natives');
   var logging = requireNative('logging');
   var schemaRegistry = requireNative('schema_registry');
   var sendRequest = require('sendRequest').sendRequest;
   var utils = require('utils');
@@ skipping 485 matching lines @@
     'hasListener',
     'hasListeners',
     'dispatchToListener',
     'dispatch',
     'addRules',
     'removeRules',
     'getRules'
   ] });
 
   // NOTE: Event is (lazily) exposed as chrome.Event from dispatcher.cc.
-  exports.Event = Event;
+  exports.$set('Event', Event);
 
-  exports.dispatchEvent = dispatchEvent;
-  exports.parseEventOptions = parseEventOptions;
-  exports.registerArgumentMassager = registerArgumentMassager;
+  exports.$set('dispatchEvent', dispatchEvent);
+  exports.$set('parseEventOptions', parseEventOptions);
+  exports.$set('registerArgumentMassager', registerArgumentMassager);