Implement ::placeholder CSS selector.

third_party/WebKit/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 161 matching lines @@
     PseudoReadWrite,
     PseudoValid,
     PseudoInvalid,
     PseudoIndeterminate,
     PseudoTarget,
     PseudoBefore,
     PseudoAfter,
     PseudoBackdrop,
     PseudoLang,
     PseudoNot,
+    PseudoPlaceholder,
     PseudoResizer,
     PseudoRoot,
     PseudoScope,
     PseudoScrollbar,
     PseudoScrollbarButton,
     PseudoScrollbarCorner,
     PseudoScrollbarThumb,
     PseudoScrollbarTrack,
     PseudoScrollbarTrackPiece,
     PseudoWindowInactive,
@@ skipping 342 matching lines @@
 }
 
 inline bool CSSSelector::isIdClassOrAttributeSelector() const {
   return isAttributeSelector() || match() == CSSSelector::Id ||
          match() == CSSSelector::Class;
 }
 
 }  // namespace blink
 
 #endif  // CSSSelector_h