Add spec links to Fullscreen API IDL

Source/core/dom/ElementFullscreen.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.
 
+// https://fullscreen.spec.whatwg.org/#api
+
 partial interface Element {
     [RuntimeEnabled=FullscreenUnprefixed] void requestFullscreen();
 
     // Mozilla version
     [LogActivity, LogAllWorlds, MeasureAs=PrefixedElementRequestFullScreen, ImplementedAs=webkitRequestFullscreen] void webkitRequestFullScreen();
 
     // W3C version
     [LogActivity, LogAllWorlds, MeasureAs=PrefixedElementRequestFullscreen] void webkitRequestFullscreen();
 
     attribute EventHandler onwebkitfullscreenchange;
     attribute EventHandler onwebkitfullscreenerror;
 };