Add priority to Notification & NotificationOptions.

Source/modules/notifications/NotificationOptions.idl

 // 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.
 
 // http://notifications.spec.whatwg.org/#api
 
 enum NotificationDirection {
     "auto",
     "ltr",
     "rtl"
 };
 
+enum NotificationPriority {
+    "lowest",
+    "low",
+    "normal",
+    "high",
+    "highest"
+};
+
 dictionary NotificationOptions {
     NotificationDirection dir = "auto";
     DOMString lang = "";
     DOMString body = "";
     DOMString tag = "";
+    [RuntimeEnabled=NotificationExperimental] NotificationPriority priority = "normal";
     DOMString icon;
     // TODO(sh919.park): vibrate should be ([Clamp] unsigned long or sequence<unsigned long>)
     (unsigned long or sequence<unsigned long>) vibrate;
     boolean silent = false;
     any data = null;
     [RuntimeEnabled=NotificationExperimental] sequence<NotificationAction> actions = [];
 };