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

Source/core/rendering/RenderLayer.h

 /*
  * Copyright (C) 2003, 2009, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 827 matching lines @@
 
     void updateFilters(const RenderStyle* oldStyle, const RenderStyle* newStyle);
 
 #if !ASSERT_DISABLED
     bool layerListMutationAllowed() const { return m_layerListMutationAllowed; }
     void setLayerListMutationAllowed(bool flag) { m_layerListMutationAllowed = flag; }
 #endif
 
     Node* enclosingElement() const;
 
+    bool isBackdrop() const { return renderer()->style()->styleType() == BACKDROP; }
     bool isInTopLayer() const;
-    bool isInTopLayerSubtree() const;
 
     virtual void reportMemoryUsage(MemoryObjectInfo*) const OVERRIDE;
 
     enum ViewportConstrainedNotCompositedReason {
         NoNotCompositedReason,
         NotCompositedForBoundsOutOfView,
         NotCompositedForNonViewContainer,
         NotCompositedForNoVisibleContent,
         NotCompositedForUnscrollableAncestors,
     };
@@ skipping 510 matching lines @@
 
 } // namespace WebCore
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showLayerTree(const WebCore::RenderLayer*);
 void showLayerTree(const WebCore::RenderObject*);
 #endif
 
 #endif // RenderLayer_h