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

extensions/renderer/resources/guest_view/web_view/web_view_events.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.
 
 // Event management for WebView.
 
 var CreateEvent = require('guestViewEvents').CreateEvent;
 var DeclarativeWebRequestSchema =
     requireNative('schema_registry').GetSchema('declarativeWebRequest');
 var EventBindings = require('event_bindings');
@@ skipping 280 matching lines @@
   }
   view.events.addScopedListener(WebRequestMessageEvent, function() {
     // Re-dispatch to subEvent's listeners.
     $Function.apply(this.dispatch, this, $Array.slice(arguments));
   }.bind(this), {instanceId: opt_webViewInstanceId || 0});
 }
 
 DeclarativeWebRequestEvent.prototype.__proto__ = EventBindings.Event.prototype;
 
 // Exports.
-exports.WebViewEvents = WebViewEvents;
+exports.$set('WebViewEvents', WebViewEvents);