Implement the ::backdrop pseudo-element for modal <dialog>.

Source/core/css/CSSSelector.h

 /*
  * Copyright (C) 1999-2003 Lars Knoll (knoll@kde.org)
  *               1999 Waldo Bastian (bastian@kde.org)
  * Copyright (C) 2004, 2006, 2007, 2008, 2009, 2010, 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
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 103 matching lines @@
             PseudoOptional,
             PseudoRequired,
             PseudoReadOnly,
             PseudoReadWrite,
             PseudoValid,
             PseudoInvalid,
             PseudoIndeterminate,
             PseudoTarget,
             PseudoBefore,
             PseudoAfter,
+            PseudoBackdrop,
             PseudoLang,
             PseudoNot,
             PseudoResizer,
             PseudoRoot,
             PseudoScope,
             PseudoScrollbar,
             PseudoScrollbarBack,
             PseudoScrollbarButton,
             PseudoScrollbarCorner,
             PseudoScrollbarForward,
@@ skipping 297 matching lines @@
     ASSERT(m_match != Tag);
     // AtomicString is really just an AtomicStringImpl* so the cast below is safe.
     // FIXME: Perhaps call sites could be changed to accept AtomicStringImpl?
     return *reinterpret_cast<const AtomicString*>(m_hasRareData ? &m_data.m_rareData->m_value : &m_data.m_value);
 }
 
 
 } // namespace WebCore
 
 #endif // CSSSelector_h