[Extensions] More bindings hardening

chrome/renderer/resources/extensions/certificate_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('certificateProvider');
 var certificateProviderInternal = require('binding').Binding.create(
     'certificateProviderInternal').generate();
 var eventBindings = require('event_bindings');
 
 var certificateProviderSchema =
@@ skipping 60 matching lines @@
         dispatch(args.slice(1).concat(reportFunc));
       });
 }
 
 handleEvent('onCertificatesRequested',
             certificateProviderInternal.reportCertificates);
 
 handleEvent('onSignDigestRequested',
             certificateProviderInternal.reportSignature);
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());