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

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

 // Copyright (c) 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 contains constants used in webview.
 
 // Container for the webview constants.
 var WebViewConstants = {
   // Attributes.
   ATTRIBUTE_ALLOWTRANSPARENCY: 'allowtransparency',
@@ skipping 19 matching lines @@
   ERROR_MSG_PERMISSION_ACTION_ALREADY_TAKEN: '<webview>: ' +
       'Permission has already been decided for this "permissionrequest" event.',
   ERROR_MSG_INVALID_PARTITION_ATTRIBUTE: '<webview>: ' +
       'Invalid partition attribute.',
   WARNING_MSG_DIALOG_REQUEST_BLOCKED: '<webview>: %1 %2 dialog was blocked.',
   WARNING_MSG_NEWWINDOW_REQUEST_BLOCKED: '<webview>: A new window was blocked.',
   WARNING_MSG_PERMISSION_REQUEST_BLOCKED: '<webview>: ' +
       'The permission request for "%1" has been denied.'
 };
 
-exports.WebViewConstants = $Object.freeze(WebViewConstants);
+exports.$set('WebViewConstants', $Object.freeze(WebViewConstants));