CSS4: Implement :placeholder-shown pseudo class

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 132 matching lines @@
         PseudoHover,
         PseudoDrag,
         PseudoFocus,
         PseudoActive,
         PseudoChecked,
         PseudoEnabled,
         PseudoFullPageMedia,
         PseudoDefault,
         PseudoDisabled,
         PseudoOptional,
+        PseudoPlaceholderShown,
         PseudoRequired,
         PseudoReadOnly,
         PseudoReadWrite,
         PseudoValid,
         PseudoInvalid,
         PseudoIndeterminate,
         PseudoTarget,
         PseudoBefore,
         PseudoAfter,
         PseudoBackdrop,
@@ skipping 316 matching lines @@
     if (m_hasRareData)
         return m_data.m_rareData->m_value;
     // AtomicString is really just a StringImpl* so the cast below is safe.
     // FIXME: Perhaps call sites could be changed to accept StringImpl?
     return *reinterpret_cast<const AtomicString*>(&m_data.m_value);
 }
 
 } // namespace blink
 
 #endif // CSSSelector_h