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

Source/core/layout/LayoutFieldset.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006 Apple Computer, Inc.
  *
  * 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.
@@ skipping 116 matching lines @@
 
 LayoutBox* LayoutFieldset::findLegend(FindLegendOption option) const
 {
     for (LayoutObject* legend = firstChild(); legend; legend = legend->nextSibling()) {
         if (option == IgnoreFloatingOrOutOfFlow && legend->isFloatingOrOutOfFlowPositioned())
             continue;
 
         if (isHTMLLegendElement(legend->node()))
             return toLayoutBox(legend);
     }
-    return 0;
+    return nullptr;
 }
 
 void LayoutFieldset::paintBoxDecorationBackground(const PaintInfo& paintInfo, const LayoutPoint& paintOffset)
 {
     FieldsetPainter(*this).paintBoxDecorationBackground(paintInfo, paintOffset);
 }
 
 void LayoutFieldset::paintMask(const PaintInfo& paintInfo, const LayoutPoint& paintOffset)
 {
     FieldsetPainter(*this).paintMask(paintInfo, paintOffset);
 }
 
 } // namespace blink