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

Source/core/dom/Element.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Peter Kelly (pmk@post.com)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 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
@@ skipping 555 matching lines @@
 
     virtual bool wasChangedSinceLastFormControlChangeEvent() const;
     virtual void setChangedSinceLastFormControlChangeEvent(bool);
     virtual void dispatchFormControlChangeEvent() { }
 
     // Used for disabled form elements; if true, prevents mouse events from being dispatched
     // to event listeners, and prevents DOMActivate events from being sent at all.
     virtual bool isDisabledFormControl() const { return false; }
 
     bool isInert() const;
-    virtual bool shouldBeReparentedUnderRenderView(const RenderStyle*) const { return isInTopLayer(); }
+    virtual bool shouldBeReparentedUnderRenderView(const RenderStyle*) const { return pseudoId() == BACKDROP || isInTopLayer(); }
 
     virtual bool childShouldCreateRenderer(const NodeRenderingContext&) const;
     bool hasPendingResources() const;
     void setHasPendingResources();
     void clearHasPendingResources();
     virtual void buildPendingResource() { };
 
     enum {
         ALLOW_KEYBOARD_INPUT = 1 << 0,
         LEGACY_MOZILLA_REQUEST = 1 << 1,
@@ skipping 468 matching lines @@
 
 inline const Attribute* ElementData::attributeItem(unsigned index) const
 {
     RELEASE_ASSERT(index < length());
     return attributeBase() + index;
 }
 
 } // namespace
 
 #endif