Implement the unprefixed the Fullscreen API

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.
 
 partial interface Element {
+    [RuntimeEnabled=FullscreenUnprefixed] void requestFullscreen();
+
     // Mozilla version
     const unsigned short ALLOW_KEYBOARD_INPUT = 1;
     [LogActivity, LogAllWorlds, MeasureAs=PrefixedElementRequestFullScreen] void webkitRequestFullScreen([Default=Undefined] optional unsigned short flags);
 
     // W3C version
     [LogActivity, LogAllWorlds, MeasureAs=PrefixedElementRequestFullscreen] void webkitRequestFullscreen();
 
     attribute EventHandler onwebkitfullscreenchange;
     attribute EventHandler onwebkitfullscreenerror;
 };