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

extensions/renderer/resources/extension_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 extension API.
 
 var binding = require('binding').Binding.create('extension');
 
 var messaging = require('messaging');
 var runtimeNatives = requireNative('runtime');
@@ skipping 90 matching lines @@
       throw new Error(sendRequestIsDisabled);
     };
     if (contextType == 'BLESSED_EXTENSION') {
       extension.onRequestExternal.addListener = function() {
         throw new Error(sendRequestIsDisabled);
       };
     }
   }
 });
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());