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

extensions/renderer/resources/image_util.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.
 
 // This function takes an object |imageSpec| with the key |path| -
 // corresponding to the internet URL to be translated - and optionally
 // |width| and |height| which are the maximum dimensions to be used when
 // converting the image.
 function loadImageData(imageSpec, callbacks) {
   var path = imageSpec.path;
@@ skipping 60 matching lines @@
 
   for (var index = 0; index < imageSpecs.length; index++) {
     loading[index] = imageSpecs[index];
     loadImageData(imageSpecs[index], {
       oncomplete: on_complete_index(index, false, loading, finished, callbacks),
       onerror: on_complete_index(index, true, loading, finished, callbacks)
     });
   }
 }
 
-exports.loadImageData = loadImageData;
-exports.loadAllImages = loadAllImages;
+exports.$set('loadImageData', loadImageData);
+exports.$set('loadAllImages', loadAllImages);