Take notification API out of experimental.

chrome/common/extensions/api/notifications.idl

 // Copyright (c) 2013 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.
 
-namespace experimental.notification {
+namespace notifications {
   enum TemplateType {
     // icon, title, message
     simple,
 
     // icon, title, message, expandedMessage, up to two buttons
     basic,
 
     // icon, title, message, expandedMessage, image, up to two buttons
     image,
 
@@ skipping 91 matching lines @@
     static void onClosed(DOMString notificationId, boolean byUser);
 
     // The user clicked in a non-button area of the notification.
     static void onClicked(DOMString notificationId);
 
     // The user pressed a button in the notification.
     static void onButtonClicked(DOMString notificationId, long buttonIndex);
   };
 
 };