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

extensions/renderer/resources/runtime_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 runtime API.
 
 var binding = require('binding').Binding.create('runtime');
 
 var messaging = require('messaging');
 var runtimeNatives = requireNative('runtime');
@@ skipping 184 matching lines @@
                                  function(name, request, callback, response) {
     if (callback) {
       var bg = runtimeNatives.GetExtensionViews(-1, 'BACKGROUND')[0] || null;
       callback(bg);
     }
   });
 
   bindDirectoryEntryCallback('getPackageDirectoryEntry', apiFunctions);
 });
 
-exports.bindDirectoryEntryCallback = bindDirectoryEntryCallback;
-exports.binding = binding.generate();
+exports.$set('bindDirectoryEntryCallback', bindDirectoryEntryCallback);
+exports.$set('binding', binding.generate());