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

extensions/renderer/resources/json_schema.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.
 
 // -----------------------------------------------------------------------------
 // NOTE: If you change this file you need to touch
 // extension_renderer_resources.grd to have your change take effect.
 // -----------------------------------------------------------------------------
 
 //==============================================================================
@@ skipping 504 matching lines @@
   });
 };
 
 /**
  * Resets errors to an empty list so you can call 'validate' again.
  */
 JSONSchemaValidator.prototype.resetErrors = function() {
   this.errors = [];
 };
 
-exports.JSONSchemaValidator = JSONSchemaValidator;
+exports.$set('JSONSchemaValidator', JSONSchemaValidator);