C++11: Replace 0 with nullptr where applicable in layout code.

Source/core/layout/LayoutProgress.cpp

 /*
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies).
  *
  * 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,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 86 matching lines @@
         m_animationStartTime = currentTime();
         m_animationTimer.startOneShot(m_animationRepeatInterval, FROM_HERE);
     } else {
         m_animationTimer.stop();
     }
 }
 
 HTMLProgressElement* LayoutProgress::progressElement() const
 {
     if (!node())
-        return 0;
+        return nullptr;
 
     if (isHTMLProgressElement(*node()))
         return toHTMLProgressElement(node());
 
     ASSERT(node()->shadowHost());
     return toHTMLProgressElement(node()->shadowHost());
 }
 
 } // namespace blink