[Extensions] More bindings hardening

chrome/renderer/resources/extensions/platform_keys_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.
 
 // Custom binding for the platformKeys API.
 
 var binding = require('binding').Binding.create('platformKeys');
 var SubtleCrypto = require('platformKeys.SubtleCrypto').SubtleCrypto;
 var getPublicKey = require('platformKeys.getPublicKey').getPublicKey;
 var internalAPI = require('platformKeys.internalAPI');
@@ skipping 45 matching lines @@
           if (chrome.runtime.lastError) {
             callback();
             return;
           }
           callback(createPublicKey(publicKey, algorithm),
                    createPrivateKey(publicKey, algorithm));
         });
       });
 });
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());