Refactor root element background painting

Source/core/style/FillLayer.cpp

 /*
  * Copyright (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 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
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 350 matching lines @@
     }
 
     return true;
 }
 
 bool FillLayer::hasOpaqueImage(const LayoutObject* layoutObject) const
 {
     if (!m_image)
         return false;
 
+    // TODO(trchen): Should check blend mode before composite mode.
     if (m_composite == CompositeClear || m_composite == CompositeCopy)
         return true;
 
     if (m_blendMode != WebBlendModeNormal)
         return false;
 
     if (m_composite == CompositeSourceOver)
         return m_image->knownToBeOpaque(layoutObject);
 
     return false;
 }
 
 bool FillLayer::hasRepeatXY() const
 {
     return m_repeatX == RepeatFill && m_repeatY == RepeatFill;
 }
 
 } // namespace blink