Ensure that privates are private.

chrome/renderer/resources/extensions/platform_keys/subtle_crypto.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 utils = require('utils');
 var internalAPI = require('platformKeys.internalAPI');
 var keyModule = require('platformKeys.Key');
 var getSpki = keyModule.getSpki;
 var KeyUsage = keyModule.KeyUsage;
 
@@ skipping 92 matching lines @@
       if (key.type != 'public')
         throw CreateInvalidAccessError();
       resolve(getSpki(key));
     } else {
       // TODO(pneubeck): It should be possible to export to format 'jwk'.
       throw CreateNotSupportedError();
     }
   });
 };
 
+function SubtleCrypto() {
+  privates(SubtleCrypto).constructPrivate(this, arguments);
+}
+utils.expose(SubtleCrypto, SubtleCryptoImpl, {
+  functions: [
+    'sign',
+    'exportKey',
+  ],
+});
+
 // Required for subclassing.
-exports.SubtleCryptoImpl = SubtleCryptoImpl
-
-exports.SubtleCrypto =
-    utils.expose('SubtleCrypto',
-                 SubtleCryptoImpl,
-                 {functions:['sign', 'exportKey']});
+exports.$set('SubtleCryptoImpl', SubtleCryptoImpl);
+exports.$set('SubtleCrypto', SubtleCrypto);