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

extensions/renderer/resources/guest_view/web_view/web_view_action_requests.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.
 
 // This module implements helper objects for the dialog, newwindow, and
 // permissionrequest <webview> events.
 
 var MessagingNatives = requireNative('messaging_natives');
 var WebViewConstants = require('webViewConstants').WebViewConstants;
 var WebViewInternal = require('webViewInternal').WebViewInternal;
@@ skipping 275 matching lines @@
 
 var WebViewActionRequests = {
   WebViewActionRequest: WebViewActionRequest,
   Dialog: Dialog,
   NewWindow: NewWindow,
   PermissionRequest: PermissionRequest,
   FullscreenPermissionRequest: FullscreenPermissionRequest
 };
 
 // Exports.
-exports.WebViewActionRequests = WebViewActionRequests;
+exports.$set('WebViewActionRequests', WebViewActionRequests);