Implement FullScreen using top layer.

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 5016 matching lines @@
     if (!element->isInTopLayer())
         return;
     size_t position = m_topLayerElements.find(element);
     ASSERT(position != kNotFound);
     m_topLayerElements.remove(position);
     element->setIsInTopLayer(false);
 }
 
 HTMLDialogElement* Document::activeModalDialog() const
 {
-    if (m_topLayerElements.isEmpty())
-        return 0;
-    return toHTMLDialogElement(m_topLayerElements.last().get());
+    for (auto it = m_topLayerElements.rbegin(); it != m_topLayerElements.rend(); ++it) {
+        Element* currentTopLayerElement = it->get();
+        if (isHTMLDialogElement(currentTopLayerElement))
+            return toHTMLDialogElement(currentTopLayerElement);
+    }
+
+    return nullptr;
 }
 
 void Document::exitPointerLock()
 {
     if (!page())
         return;
     if (Element* target = page()->pointerLockController().element()) {
         if (target->document() != this)
             return;
     }
@@ skipping 691 matching lines @@
 #ifndef NDEBUG
 using namespace blink;
 void showLiveDocumentInstances()
 {
     Document::WeakDocumentSet& set = Document::liveDocumentSet();
     fprintf(stderr, "There are %u documents currently alive:\n", set.size());
     for (Document* document : set)
         fprintf(stderr, "- Document %p URL: %s\n", document, document->url().string().utf8().data());
 }
 #endif