Restore the HTMLVideoElement-specific prefixed fullscreen API behind a flag

Source/core/html/HTMLMediaElement.cpp

 /*
  * 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 3171 matching lines @@
 }
 
 void HTMLMediaElement::enterFullscreen()
 {
     WTF_LOG(Media, "HTMLMediaElement::enterFullscreen");
 
     if (document().settings() && document().settings()->fullScreenEnabled())
         FullscreenElementStack::from(document()).requestFullScreenForElement(this, 0, FullscreenElementStack::ExemptIFrameAllowFullScreenRequirement);
 }
 
+void HTMLMediaElement::exitFullscreen()
+{
+    WTF_LOG(Media, "HTMLMediaElement::exitFullscreen");
+
+    if (document().settings() && document().settings()->fullScreenEnabled() && isFullscreen())
+        FullscreenElementStack::from(document()).webkitCancelFullScreen();
+}
+
 void HTMLMediaElement::didBecomeFullscreenElement()
 {
     if (hasMediaControls())
         mediaControls()->enteredFullscreen();
     if (RuntimeEnabledFeatures::overlayFullscreenVideoEnabled() && isHTMLVideoElement(*this))
         document().renderView()->compositor()->setCompositingLayersNeedRebuild();
 }
 
 void HTMLMediaElement::willStopBeingFullscreenElement()
 {
@@ skipping 443 matching lines @@
 {
     m_mediaSource->setWebMediaSourceAndOpen(adoptPtr(webMediaSource));
 }
 
 bool HTMLMediaElement::isInteractiveContent() const
 {
     return fastHasAttribute(controlsAttr);
 }
 
 }