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

extensions/renderer/resources/guest_view/web_view/web_view_request_custom_bindings.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.
 
 // Custom binding for the webViewRequest API.
 
 var binding = require('binding').Binding.create('webViewRequest');
 
 var declarativeWebRequestSchema =
     requireNative('schema_registry').GetSchema('declarativeWebRequest');
@@ skipping 34 matching lines @@
     var typeId = typeSchema.id.replace('declarativeWebRequest.', '');
     var action = function(typeId) {
       return function(parameters) {
         setupInstance(this, parameters, typeId);
       };
     }(typeId);
     webViewRequest[typeId] = action;
   }
 });
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());