[Extensions] More bindings hardening

chrome/renderer/resources/extensions/enterprise_platform_keys_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 binding for the enterprise.platformKeys API.
 
 // The platformKeys API consists of two major parts:
 //   - the certificate management.
 //   - the key generation and crypto operations and
 // The former is implemented without custom binding as static functions.
@@ skipping 11 matching lines @@
   var apiFunctions = api.apiFunctions;
 
   var ret = apiFunctions.setHandleRequest('getTokens', function(callback) {
     internalAPI.getTokens(function(tokenIds) {
       callback($Array.map(tokenIds,
                           function(tokenId) { return new Token(tokenId); }));
     });
   });
 });
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());