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

Source/core/layout/LayoutButton.h

 /*
  * Copyright (C) 2005 Apple Computer
  *
  * 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 22 matching lines @@
 public:
     explicit LayoutButton(Element*);
     virtual ~LayoutButton();
 
     virtual const char* name() const override { return "LayoutButton"; }
     virtual bool isOfType(LayoutObjectType type) const override { return type == LayoutObjectLayoutButton || LayoutFlexibleBox::isOfType(type); }
 
     virtual bool canBeSelectionLeaf() const override { return node() && node()->hasEditableStyle(); }
     virtual bool canCollapseAnonymousBlockChild() const override { return true; }
 
-    virtual void addChild(LayoutObject* newChild, LayoutObject *beforeChild = 0) override;
+    virtual void addChild(LayoutObject* newChild, LayoutObject *beforeChild = nullptr) override;
     virtual void removeChild(LayoutObject*) override;
     virtual void removeLeftoverAnonymousBlock(LayoutBlock*) override { }
     virtual bool createsAnonymousWrapper() const override { return true; }
 
     virtual bool hasControlClip() const override;
     virtual LayoutRect controlClipRect(const LayoutPoint&) const override;
 
     virtual int baselinePosition(FontBaseline, bool firstLine, LineDirectionMode, LinePositionMode) const override;
 
 private:
     virtual void updateAnonymousChildStyle(const LayoutObject& child, ComputedStyle& childStyle) const override;
 
     virtual bool hasLineIfEmpty() const override { return isHTMLInputElement(node()); }
 
     LayoutBlock* m_inner;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutButton, isLayoutButton());
 
 } // namespace blink
 
 #endif // LayoutButton_h