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

Source/core/rendering/style/RenderStyle.h

 /*
  * Copyright (C) 2000 Lars Knoll (knoll@kde.org)
  *           (C) 2000 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2003, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *
  * 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 223 matching lines @@
         unsigned _floating : 2; // EFloat
         unsigned _table_layout : 1; // ETableLayout
 
         unsigned _unicodeBidi : 3; // EUnicodeBidi
         // 31 bits
         unsigned _page_break_before : 2; // EPageBreak
         unsigned _page_break_after : 2; // EPageBreak
         unsigned _page_break_inside : 2; // EPageBreak
 
         unsigned _styleType : 6; // PseudoId
-        unsigned _pseudoBits : 7;
+        unsigned _pseudoBits : 8;
         unsigned explicitInheritance : 1; // Explicitly inherits a non-inherited property
         unsigned unique : 1; // Style can not be shared.
         unsigned emptyState : 1;
         unsigned firstChildState : 1;
         unsigned lastChildState : 1;
 
         bool affectedByHover() const { return _affectedByHover; }
         void setAffectedByHover(bool value) { _affectedByHover = value; }
         bool affectedByActive() const { return _affectedByActive; }
         void setAffectedByActive(bool value) { _affectedByActive = value; }
         bool affectedByDrag() const { return _affectedByDrag; }
         void setAffectedByDrag(bool value) { _affectedByDrag = value; }
         bool isLink() const { return _isLink; }
         void setIsLink(bool value) { _isLink = value; }
     private:
         unsigned _affectedByHover : 1;
         unsigned _affectedByActive : 1;
         unsigned _affectedByDrag : 1;
         unsigned _isLink : 1;
         // If you add more style bits here, you will also need to update RenderStyle::copyNonInheritedFrom()
-        // 59 bits
+        // 60 bits
     } noninherited_flags;
 
 // !END SYNC!
 
 protected:
     void setBitDefaults()
     {
         inherited_flags._empty_cells = initialEmptyCells();
         inherited_flags._caption_side = initialCaptionSide();
         inherited_flags._list_style_type = initialListStyleType();
@@ skipping 1478 matching lines @@
     if (compareEqual(rareInheritedData->m_textOrientation, textOrientation))
         return false;
 
     rareInheritedData.access()->m_textOrientation = textOrientation;
     return true;
 }
 
 } // namespace WebCore
 
 #endif // RenderStyle_h