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

extensions/renderer/resources/guest_view/extension_options/extension_options_constants.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.
 
 // This module contains constants used in extensionoptions.
 
 // Container for the extensionview constants.
 var ExtensionOptionsConstants = {
   // Attributes.
   ATTRIBUTE_EXTENSION: 'extension'
 };
 
-exports.ExtensionOptionsConstants = $Object.freeze(ExtensionOptionsConstants);
+exports.$set('ExtensionOptionsConstants',
+             $Object.freeze(ExtensionOptionsConstants));