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

extensions/renderer/resources/mime_handler_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 mime handler API.
  */
 
 var binding = require('binding').Binding.create('mimeHandlerPrivate');
 
@@ skipping 54 matching lines @@
     return streamInfoPromise.then(constructStreamInfoDict);
   });
 
   apiFunctions.setHandleRequestWithPromise('abortStream', function() {
     return servicePromise.then(function(service) {
       return service.abortStream().then(function() {});
     }).catch(throwNoStreamError);
   });
 });
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());