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

Source/core/dom/Document.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
@@ skipping 1035 matching lines @@
     void clearStyleResolver();
     void notifySeamlessChildDocumentsOfStylesheetUpdate() const;
 
     bool inStyleRecalc() { return m_inStyleRecalc; }
 
     // Return a Locale for the default locale if the argument is null or empty.
     Locale& getCachedLocale(const AtomicString& locale = nullAtom);
 
     DocumentTimeline* timeline() { return m_timeline.get(); }
 
-    void addToTopLayer(Element*);
+    void addToTopLayer(Element*, const Element* before = 0);
     void removeFromTopLayer(Element*);
     const Vector<RefPtr<Element> >& topLayerElements() const { return m_topLayerElements; }
     Element* activeModalDialog() const { return !m_topLayerElements.isEmpty() ? m_topLayerElements.last().get() : 0; }
 
     const Document* templateDocument() const;
     Document* ensureTemplateDocument();
     void setTemplateDocumentHost(Document* templateDocumentHost) { m_templateDocumentHost = templateDocumentHost; }
     Document* templateDocumentHost() { return m_templateDocumentHost; }
 
     void didAssociateFormControl(Element*);
@@ skipping 375 matching lines @@
 inline bool Node::isDocumentNode() const
 {
     return this == documentInternal();
 }
 
 Node* eventTargetNodeForDocument(Document*);
 
 } // namespace WebCore
 
 #endif // Document_h