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

extensions/renderer/resources/entry_id_manager.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 fileSystemNatives = requireNative('file_system_natives');
 
 var nameToIds = {};
 var idsToEntries = {};
 
 function computeName(entry) {
@@ skipping 29 matching lines @@
   // If an entry has not been registered, compute its id and register it.
   id = computeId(entry);
   registerEntry(id, entry);
   return id;
 }
 
 function getEntryById(id) {
   return idsToEntries[id];
 }
 
-exports.registerEntry = registerEntry;
-exports.getEntryId = getEntryId;
-exports.getEntryById = getEntryById;
+exports.$set('registerEntry', registerEntry);
+exports.$set('getEntryId', getEntryId);
+exports.$set('getEntryById', getEntryById);