Run the DOMStorageBrowserTest.SanityCheck against the mojo based implementation.

content/test/data/dom_storage/sanity_check.js

 // Copyright (c) 2012 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.
 
 function startTestSoon() {
   window.setTimeout(test, 0);
 }
 
 function test() {
   try {
     debug('Checking window.localStorage');
     sanityCheck(window.localStorage);
     debug('Checking window.sessionStorage');
     sanityCheck(window.sessionStorage);
     done();
   } catch(e) {
     fail(e);
   }
 }
 
 function sanityCheck(storage) {
+  storage["preload"] = "done";
+  checkEqual("done", storage["preload"],
+             "storage['preload'] != 'done' after addition");
+
   storage.clear();
 
   checkEqual(0, storage.length,
              "storage.length != 0 at start");
   checkEqual(null, storage.getItem("foo"),
              "getItem('foo') != null prior to addition");
   checkEqual(null, storage.key(0),
              "key(0) != null prior to addition");
 
   storage.setItem("foo", "bar");
@@ skipping 43 matching lines @@
 }
 
 function checkEqual(lhs, rhs, errorMessage) {
   if (lhs !== rhs)
     throw errorMessage;
 }
 
 function makeLargeString(minimumSize) {
   return Array(minimumSize).join("X");
 }