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

extensions/renderer/resources/app_window_custom_bindings.js

 // Copyright (c) 2012 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 app_window API.
 
 var appWindowNatives = requireNative('app_window_natives');
 var runtimeNatives = requireNative('runtime');
 var Binding = require('binding').Binding;
 var Event = require('event_bindings').Event;
@@ skipping 386 matching lines @@
     // From the perspective of the API, null is used to reset constraints.
     // We need to convert this to 0 because a value of null is interpreted
     // the same as undefined in the browser and leaves the constraint unchanged.
     if (value === null)
       constraints[key] = 0;
   });
 
   currentWindowInternal.setSizeConstraints(boundsType, constraints);
 }
 
-exports.binding = appWindow.generate();
-exports.onAppWindowClosed = onAppWindowClosed;
-exports.updateAppWindowProperties = updateAppWindowProperties;
-exports.appWindowShownForTests = onAppWindowShownForTests;
+exports.$set('binding', appWindow.generate());
+exports.$set('onAppWindowClosed', onAppWindowClosed);
+exports.$set('updateAppWindowProperties', updateAppWindowProperties);
+exports.$set('appWindowShownForTests', onAppWindowShownForTests);