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

extensions/renderer/resources/printer_provider_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.
 
 var binding = require('binding').Binding.create('printerProvider');
 var printerProviderInternal = require('binding').Binding.create(
     'printerProviderInternal').generate();
 var eventBindings = require('event_bindings');
 var blobNatives = requireNative('blob_natives');
 
@@ skipping 102 matching lines @@
             printerProviderInternal.reportPrinterCapability);
 
 handleEvent('onPrintRequested',
             createPrintRequestBlobArguments,
             printerProviderInternal.reportPrintResult);
 
 handleEvent('onGetUsbPrinterInfoRequested',
             function(args, callback) { callback(true); },
             printerProviderInternal.reportUsbPrinterInfo);
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());