Implement the unprefixed the Fullscreen API

Source/core/dom/ElementFullscreen.h

 // 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.
 
 #ifndef ElementFullscreen_h
 #define ElementFullscreen_h
 
 #include "core/events/EventTarget.h"
 
 namespace blink {
 
 class Element;
 
 class ElementFullscreen {
 public:
+    static void requestFullscreen(Element&);
+
     // Mozilla version
     static const unsigned short ALLOW_KEYBOARD_INPUT = 1;
     static void webkitRequestFullScreen(Element&, unsigned short flags);
 
     // W3C version
     static void webkitRequestFullscreen(Element&);
 
     DEFINE_STATIC_ATTRIBUTE_EVENT_LISTENER(webkitfullscreenchange);
     DEFINE_STATIC_ATTRIBUTE_EVENT_LISTENER(webkitfullscreenerror);
 };
 
 } // namespace blink
 
 #endif // ElementFullscreen_h