Extract "fullscreen element ready check" from requestFullscreen()

Source/core/dom/FullscreenElementStack.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
@@ skipping 85 matching lines @@
 
     virtual void trace(Visitor*) OVERRIDE;
 
 private:
     static FullscreenElementStack* fromIfExistsSlow(Document&);
 
     explicit FullscreenElementStack(Document&);
 
     Document* document();
 
+    static bool elementReady(Element&, RequestType);
+
     void clearFullscreenElementStack();
     void popFullscreenElementStack();
     void pushFullscreenElementStack(Element&, RequestType);
 
     void enqueueChangeEvent(Document&, RequestType);
     void enqueueErrorEvent(Element&, RequestType);
     void eventQueueTimerFired(Timer<FullscreenElementStack>*);
 
     void fullScreenElementRemoved();
 
@@ skipping 18 matching lines @@
 inline FullscreenElementStack* FullscreenElementStack::fromIfExists(Document& document)
 {
     if (!document.hasFullscreenElementStack())
         return 0;
     return fromIfExistsSlow(document);
 }
 
 } // namespace blink
 
 #endif // FullscreenElementStack_h