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

Source/core/layout/LayoutPart.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2000 Simon Hausmann <hausmann@kde.org>
  *           (C) 2000 Stefan Schimanski (1Stein@gmx.de)
  * Copyright (C) 2004, 2005, 2006, 2009 Apple Inc. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. 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
@@ skipping 84 matching lines @@
 }
 
 Widget* LayoutPart::widget() const
 {
     // Plugin widgets are stored in their DOM node. This includes HTMLAppletElement.
     Element* element = toElement(node());
 
     if (element && element->isFrameOwnerElement())
         return toHTMLFrameOwnerElement(element)->ownedWidget();
 
-    return 0;
+    return nullptr;
 }
 
 DeprecatedPaintLayerType LayoutPart::layerTypeRequired() const
 {
     DeprecatedPaintLayerType type = LayoutReplaced::layerTypeRequired();
     if (type != NoDeprecatedPaintLayer)
         return type;
     return ForcedDeprecatedPaintLayer;
 }
 
@@ skipping 203 matching lines @@
     if (widget->frameRect() == newFrame)
         return false;
 
     RefPtr<LayoutPart> protector(this);
     RefPtrWillBeRawPtr<Node> protectedNode(node());
     widget->setFrameRect(newFrame);
     return widget->frameRect().size() != newFrame.size();
 }
 
 }