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

Source/core/rendering/style/RenderStyleConstants.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 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 52 matching lines @@
 enum StyleDifferenceContextSensitiveProperty {
     ContextSensitivePropertyNone = 0,
     ContextSensitivePropertyTransform = (1 << 0),
     ContextSensitivePropertyOpacity = (1 << 1),
     ContextSensitivePropertyFilter = (1 << 2)
 };
 
 // Static pseudo styles. Dynamic ones are produced on the fly.
 enum PseudoId {
     // The order must be NOP ID, public IDs, and then internal IDs.
-    NOPSEUDO, FIRST_LINE, FIRST_LETTER, BEFORE, AFTER, SELECTION, FIRST_LINE_INHERITED, SCROLLBAR,
+    // If you add or remove a public ID, you must update _pseudoBits in RenderStyle.
+    NOPSEUDO, FIRST_LINE, FIRST_LETTER, BEFORE, AFTER, BACKDROP, SELECTION, FIRST_LINE_INHERITED, SCROLLBAR,
     // Internal IDs follow:
     SCROLLBAR_THUMB, SCROLLBAR_BUTTON, SCROLLBAR_TRACK, SCROLLBAR_TRACK_PIECE, SCROLLBAR_CORNER, RESIZER,
     INPUT_LIST_BUTTON,
     AFTER_LAST_INTERNAL_PSEUDOID,
     FULL_SCREEN, FULL_SCREEN_DOCUMENT, FULL_SCREEN_ANCESTOR,
     FIRST_PUBLIC_PSEUDOID = FIRST_LINE,
     FIRST_INTERNAL_PSEUDOID = SCROLLBAR_THUMB,
     PUBLIC_PSEUDOID_MASK = ((1 << FIRST_INTERNAL_PSEUDOID) - 1) & ~((1 << FIRST_PUBLIC_PSEUDOID) - 1)
 };
 
@@ skipping 416 matching lines @@
 // Reasonable maximum to prevent insane font sizes from causing crashes on some platforms (such as Windows).
 static const float maximumAllowedFontSize = 1000000.0f;
 
 #if ENABLE(CSS3_TEXT)
 enum TextIndentLine { TextIndentFirstLine, TextIndentEachLine };
 #endif
 
 } // namespace WebCore
 
 #endif // RenderStyleConstants_h