Restore the HTMLVideoElement-specific prefixed fullscreen API behind a flag

Source/core/html/HTMLMediaElement.h

 /*
  * Copyright (C) 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 215 matching lines @@
     // Both Node (via HTMLElement) and ActiveDOMObject define this method, which
     // causes an ambiguity error at compile time. This class's constructor
     // ensures that both implementations return document, so return the result
     // of one of them here.
     virtual ExecutionContext* executionContext() const OVERRIDE FINAL { return HTMLElement::executionContext(); }
 
     bool hasSingleSecurityOrigin() const { return !m_player || m_player->hasSingleSecurityOrigin(); }
 
     bool isFullscreen() const;
     void enterFullscreen();
+    void exitFullscreen();
 
     bool hasClosedCaptions() const;
     bool closedCaptionsVisible() const;
     void setClosedCaptionsVisible(bool);
 
     MediaControls* mediaControls() const;
 
     void sourceWasRemoved(HTMLSourceElement*);
     void sourceWasAdded(HTMLSourceElement*);
 
@@ skipping 307 matching lines @@
 inline bool isHTMLMediaElement(const HTMLElement& element)
 {
     return isHTMLAudioElement(element) || isHTMLVideoElement(element);
 }
 
 DEFINE_HTMLELEMENT_TYPE_CASTS_WITH_FUNCTION(HTMLMediaElement);
 
 } //namespace
 
 #endif