Take the Extension Storage API out of experimental.

chrome/common/extensions/docs/examples/api/storage/stylizr/popup.js

-// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// 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.
 
 // Store settings in the synchronized repository.
-var storage = chrome.experimental.storage.sync;
+var storage = chrome.storage.sync;
 var message = document.querySelector('#message');
 
 // Check if there is CSS specified.
 storage.get('css', function(items) {
   console.log(items);
   // If there is CSS specified, inject it into the page.
   if (items.css) {
     chrome.tabs.insertCSS(null, {code: items.css}, function() {
       if (chrome.extension.lastError) {
         message.innerText = 'Not allowed to inject CSS into special page.';
       } else {
         message.innerText = 'Injected style!';
       }
     });
   } else {
     var optionsUrl = chrome.extension.getURL('options.html');
     message.innerHTML = 'Set a style in the <a target="_blank" href="' +
         optionsUrl + '">options page</a> first.';
   }
 });