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

extensions/renderer/resources/mojo_private_custom_bindings.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.
 
 /**
  * Custom bindings for the mojoPrivate API.
  */
 
 let binding = require('binding').Binding.create('mojoPrivate');
 
 binding.registerCustomHook(function(bindingsAPI) {
   let apiFunctions = bindingsAPI.apiFunctions;
 
   apiFunctions.setHandleRequest('define', function(name, deps, factory) {
     define(name, deps || [], factory);
   });
 
   apiFunctions.setHandleRequest('requireAsync', function(moduleName) {
     return requireAsync(moduleName);
   });
 });
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());