Implement FullScreen using top layer.

Source/core/css/resolver/StyleResolver.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2004-2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2006, 2007 Nicholas Shanks (webkit@nickshanks.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007, 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (c) 2011, Code Aurora Forum. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. All rights reserved.
@@ skipping 656 matching lines @@
     // of the ancestors are first letter.
     if (pseudoId < FIRST_INTERNAL_PSEUDOID && pseudoId != FIRST_LETTER && !parentLayoutObject->style()->hasPseudoStyle(pseudoId))
         return nullptr;
 
     if (pseudoId == BACKDROP && !parent.isInTopLayer())
         return nullptr;
 
     if (pseudoId == FIRST_LETTER && (parent.isSVGElement() || !FirstLetterPseudoElement::firstLetterTextLayoutObject(parent)))
         return nullptr;
 
-    if (!canHaveGeneratedChildren(*parentLayoutObject))
+    if (pseudoId != BACKDROP && !canHaveGeneratedChildren(*parentLayoutObject))
         return nullptr;
 
     ComputedStyle* parentStyle = parentLayoutObject->mutableStyle();
     if (ComputedStyle* cachedStyle = parentStyle->getCachedPseudoStyle(pseudoId)) {
         if (!pseudoElementLayoutObjectIsNeeded(cachedStyle))
             return nullptr;
         return createPseudoElement(&parent, pseudoId);
     }
 
     StyleResolverState state(document(), &parent, parentStyle);
@@ skipping 799 matching lines @@
     visitor->trace(m_watchedSelectorsRules);
     visitor->trace(m_treeBoundaryCrossingRules);
     visitor->trace(m_styleResourceLoader);
     visitor->trace(m_styleSharingLists);
     visitor->trace(m_pendingStyleSheets);
     visitor->trace(m_document);
 #endif
 }
 
 } // namespace blink