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

extensions/renderer/resources/guest_view/guest_view_events.js

 // Copyright 2015 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 GuestViewContainers.
 
 var EventBindings = require('event_bindings');
 var GuestViewInternalNatives = requireNative('guest_view_internal');
 var MessagingNatives = requireNative('messaging_natives');
 
@@ skipping 156 matching lines @@
       return;
     }
     return $Function.apply(func, view.events, $Array.slice(arguments));
   };
 };
 
 // Implemented by the derived event manager, if one exists.
 GuestViewEvents.prototype.getEvents = function() { return {}; };
 
 // Exports.
-exports.GuestViewEvents = GuestViewEvents;
-exports.CreateEvent = CreateEvent;
+exports.$set('GuestViewEvents', GuestViewEvents);
+exports.$set('CreateEvent', CreateEvent);