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

Source/core/layout/BidiRun.h

 /**
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Andrew Wellington (proton@wiretapped.net)
  *
  * 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 19 matching lines @@
 
 namespace blink {
 
 class BidiContext;
 class InlineBox;
 
 struct BidiRun : BidiCharacterRun {
     BidiRun(int start, int stop, LayoutObject* object, BidiContext* context, WTF::Unicode::Direction dir)
         : BidiCharacterRun(start, stop, context, dir)
         , m_object(object)
-        , m_box(0)
+        , m_box(nullptr)
     {
         // Stored in base class to save space.
         m_hasHyphen = false;
     }
 
     BidiRun* next() { return static_cast<BidiRun*>(m_next); }
     LayoutObject* object() { return m_object; }
 
 public:
     LayoutObject* m_object;
     InlineBox* m_box;
 };
 
 }
 
 #endif // BidiRun_h