Add spec links to Fullscreen API IDL

Source/core/dom/DocumentFullscreen.idl

 /*
  * Copyright (C) 2006, 2007, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006, 2007 Samuel Weinig <sam@webkit.org>
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Library General Public License for more details.
  *
  * You should have received a copy of the GNU Library General Public License
  * along with this library; see the file COPYING.LIB.  If not, write to
  * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  * Boston, MA 02110-1301, USA.
  */
 
+// https://fullscreen.spec.whatwg.org/#api
+
 partial interface Document {
     [RuntimeEnabled=FullscreenUnprefixed] readonly attribute boolean fullscreenEnabled;
     [RuntimeEnabled=FullscreenUnprefixed] readonly attribute Element? fullscreenElement;
 
     [RuntimeEnabled=FullscreenUnprefixed] void exitFullscreen();
 
     [RuntimeEnabled=FullscreenUnprefixed] attribute EventHandler onfullscreenchange;
     [RuntimeEnabled=FullscreenUnprefixed] attribute EventHandler onfullscreenerror;
 
     // Mozilla version
     [MeasureAs=PrefixedDocumentIsFullscreen, ImplementedAs=webkitCurrentFullScreenElement] readonly attribute boolean webkitIsFullScreen;
     [MeasureAs=PrefixedDocumentCurrentFullScreenElement] readonly attribute Element webkitCurrentFullScreenElement;
     [MeasureAs=PrefixedDocumentCancelFullScreen, ImplementedAs=exitFullscreen] void webkitCancelFullScreen();
 
     // W3C version
     [MeasureAs=PrefixedDocumentFullscreenEnabled, ImplementedAs=fullscreenEnabled] readonly attribute boolean webkitFullscreenEnabled;
     [MeasureAs=PrefixedDocumentFullscreenElement, ImplementedAs=fullscreenElement] readonly attribute Element webkitFullscreenElement;
     [MeasureAs=PrefixedDocumentExitFullscreen, ImplementedAs=exitFullscreen] void webkitExitFullscreen();
 
     attribute EventHandler onwebkitfullscreenchange;
     attribute EventHandler onwebkitfullscreenerror;
 };