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

Source/core/css/PseudoStyleRequest.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 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.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 20 matching lines @@
 
 class PseudoStyleRequest {
 public:
     PseudoStyleRequest(PseudoId pseudoId, RenderScrollbar* scrollbar = 0, ScrollbarPart scrollbarPart = NoPart)
         : pseudoId(pseudoId)
         , scrollbarPart(scrollbarPart)
         , scrollbar(scrollbar)
     {
     }
 
+    // The spec disallows inheritance for ::backdrop.
+    bool allowsInheritance(const RenderStyle* parentStyle) const
+    {
+        return parentStyle && pseudoId != BACKDROP;
+    }
+
     PseudoId pseudoId;
     ScrollbarPart scrollbarPart;
     RenderScrollbar* scrollbar;
 };
 
 } // namespace WebCore
 
 #endif // PseudoStyleRequest_h