Remove Apple's unused implementation of private browsing from WebCore

Source/WebCore/dom/Element.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Peter Kelly (pmk@post.com)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2013 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
@@ skipping 465 matching lines @@
     LayoutSize minimumSizeForResizing() const;
     void setMinimumSizeForResizing(const LayoutSize&);
 
     // Use Document::registerForDocumentActivationCallbacks() to subscribe to these
     virtual void documentWillSuspendForPageCache() { }
     virtual void documentDidResumeFromPageCache() { }
 
     // Use Document::registerForMediaVolumeCallbacks() to subscribe to this
     virtual void mediaVolumeDidChange() { }
 
-    // Use Document::registerForPrivateBrowsingStateChangedCallbacks() to subscribe to this.
-    virtual void privateBrowsingStateDidChange() { }
-
     virtual void didBecomeFullscreenElement() { }
     virtual void willStopBeingFullscreenElement() { }
 
 #if ENABLE(VIDEO_TRACK)
     virtual void captionPreferencesChanged() { }
 #endif
 
     bool isFinishedParsingChildren() const { return isParsingChildrenFinished(); }
     virtual void finishParsingChildren();
     virtual void beginParsingChildren();
@@ skipping 494 matching lines @@
 {
     ASSERT_WITH_SECURITY_IMPLICATION(index < length());
     if (m_isUnique)
         return &static_cast<const UniqueElementData*>(this)->m_attributeVector.at(index);
     return &static_cast<const ShareableElementData*>(this)->m_attributeArray[index];
 }
 
 } // namespace
 
 #endif