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

extensions/renderer/resources/guest_view/web_view/web_view.js

 // Copyright (c) 2012 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 WebView (<webview>) as a custom element that wraps a
 // BrowserPlugin object element. The object element is hidden within
 // the shadow DOM of the WebView element.
 
 var DocumentNatives = requireNative('document_natives');
 var GuestView = require('guestView').GuestView;
@@ skipping 205 matching lines @@
     this.element.webkitRequestFullScreen();
   }.bind(this));
 };
 
 // Implemented when the ChromeWebView API is available.
 WebViewImpl.prototype.maybeSetupContextMenus = function() {};
 
 GuestViewContainer.registerElement(WebViewImpl);
 
 // Exports.
-exports.WebViewImpl = WebViewImpl;
+exports.$set('WebViewImpl', WebViewImpl);