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

extensions/renderer/resources/app_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 chrome.app.runtime API.
 
 var binding = require('binding').Binding.create('app.runtime');
 
 var AppViewGuestInternal =
     require('binding').Binding.create('appViewGuestInternal').generate();
@@ skipping 53 matching lines @@
         itemLoaded(fileError);
       });
     });
   } else {
     // Default case. This currently covers an onLaunched corresponding to
     // url_handlers in the app's manifest.
     dispatch([launchData]);
   }
 });
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());