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

extensions/renderer/resources/set_icon.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.
 
 var SetIconCommon = requireNative('setIcon').SetIconCommon;
 var sendRequest = require('sendRequest').sendRequest;
 
 function loadImagePath(path, iconSize, callback) {
   var img = new Image();
   img.onerror = function() {
@@ skipping 115 matching lines @@
             delete details.path;
             callback(SetIconCommon(details));
             return;
       });
     }
     return;
   }
   throw new Error('Either the path or imageData property must be specified.');
 }
 
-exports.setIcon = setIcon;
+exports.$set('setIcon', setIcon);