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

Source/core/dom/Document.cpp

 /*
  * 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, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2008, 2009, 2011, 2012 Google Inc. All rights reserved.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) Research In Motion Limited 2010-2011. All rights reserved.
@@ skipping 4648 matching lines @@
 
 void Document::enqueuePopstateEvent(PassRefPtr<SerializedScriptValue> stateObject)
 {
     if (!ContextFeatures::pushStateEnabled(this))
         return;
 
     // FIXME: https://bugs.webkit.org/show_bug.cgi?id=36202 Popstate event needs to fire asynchronously
     dispatchWindowEvent(PopStateEvent::create(stateObject, domWindow() ? domWindow()->history() : 0));
 }
 
-void Document::addToTopLayer(Element* element)
+void Document::addToTopLayer(Element* element, const Element* before)
 {
     if (element->isInTopLayer())
         return;
+
     ASSERT(!m_topLayerElements.contains(element));
-    m_topLayerElements.append(element);
+    ASSERT(!before || m_topLayerElements.contains(before));
+    if (before) {
+        size_t beforePosition = m_topLayerElements.find(before);
+        m_topLayerElements.insert(beforePosition, element);
+    } else {
+        m_topLayerElements.append(element);
+    }
     element->setIsInTopLayer(true);
 }
 
 void Document::removeFromTopLayer(Element* element)
 {
     if (!element->isInTopLayer())
         return;
     size_t position = m_topLayerElements.find(element);
     ASSERT(position != notFound);
     m_topLayerElements.remove(position);
@@ skipping 545 matching lines @@
 {
     return DocumentLifecycleNotifier::create(this);
 }
 
 DocumentLifecycleNotifier* Document::lifecycleNotifier()
 {
     return static_cast<DocumentLifecycleNotifier*>(ScriptExecutionContext::lifecycleNotifier());
 }
 
 } // namespace WebCore