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

extensions/renderer/resources/serial_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 bindings for the Serial API.
  *
  * The bindings are implemented by asynchronously delegating to the
  * serial_service module. The functions that apply to a particular connection
  * are delegated to the appropriate method on the Connection object specified by
@@ skipping 96 matching lines @@
     }).then(function(connections) {
       var promises = [];
       for (var connection of connections.values()) {
         promises.push(connection.getInfo());
       }
       return Promise.all(promises);
     });
   });
 });
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());