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

Source/core/layout/LayoutMenuList.h

 /*
  * This file is part of the select element layoutObject in WebCore.
  *
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies).
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011 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.
@@ skipping 46 matching lines @@
 
 protected:
     virtual void willBeDestroyed() override;
 
 private:
     HTMLSelectElement* selectElement() const;
 
     virtual bool isOfType(LayoutObjectType type) const override { return type == LayoutObjectMenuList || LayoutFlexibleBox::isOfType(type); }
     virtual bool isChildAllowed(LayoutObject*, const ComputedStyle&) const override;
 
-    virtual void addChild(LayoutObject* newChild, LayoutObject* beforeChild = 0) override;
+    virtual void addChild(LayoutObject* newChild, LayoutObject* beforeChild = nullptr) override;
     virtual void removeChild(LayoutObject*) override;
     virtual bool createsAnonymousWrapper() const override { return true; }
 
     virtual void updateFromElement() override;
 
     virtual LayoutRect controlClipRect(const LayoutPoint&) const override;
     virtual bool hasControlClip() const override { return true; }
 
     virtual void computeIntrinsicLogicalWidths(LayoutUnit& minLogicalWidth, LayoutUnit& maxLogicalWidth) const override;
 
@@ skipping 65 matching lines @@
 
     // TODO(tkent): Use FRIEND_TEST macro provided by gtest_prod.h
     friend class LayoutMenuListTest_ElementRectRelativeToViewport_Test;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutMenuList, isMenuList());
 
 }
 
 #endif