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

Source/core/layout/LayoutListMarker.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2009 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 53 matching lines @@
     LayoutListMarker(LayoutListItem*);
 
     virtual void computePreferredLogicalWidths() override;
 
     virtual bool isOfType(LayoutObjectType type) const override { return type == LayoutObjectListMarker || LayoutBox::isOfType(type); }
 
     virtual void paint(const PaintInfo&, const LayoutPoint&) override;
 
     virtual void layout() override;
 
-    virtual void imageChanged(WrappedImagePtr, const IntRect* = 0) override;
+    virtual void imageChanged(WrappedImagePtr, const IntRect* = nullptr) override;
 
     virtual InlineBox* createInlineBox() override;
 
     virtual LayoutUnit lineHeight(bool firstLine, LineDirectionMode, LinePositionMode = PositionOnContainingLine) const override;
     virtual int baselinePosition(FontBaseline, bool firstLine, LineDirectionMode, LinePositionMode = PositionOnContainingLine) const override;
 
     bool isText() const { return !isImage(); }
 
     virtual void setSelectionState(SelectionState) override;
     virtual LayoutRect selectionRectForPaintInvalidation(const LayoutBoxModelObject* paintInvalidationContainer) const override;
     virtual bool canBeSelectionLeaf() const override { return true; }
 
     void updateMargins();
     void updateContent();
 
     virtual void styleWillChange(StyleDifference, const ComputedStyle& newStyle) override;
     virtual void styleDidChange(StyleDifference, const ComputedStyle* oldStyle) override;
 
     String m_text;
     RefPtr<StyleImage> m_image;
     LayoutListItem* m_listItem;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutListMarker, isListMarker());
 
 } // namespace blink
 
 #endif // LayoutListMarker_h