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

Source/core/layout/LayoutGrid.h

 /*
  * Copyright (C) 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 69 matching lines @@
         return m_gridItemsIndexesMap.get(layoutBox);
     }
 
 private:
     typedef void (GridTrack::* AccumulatorGrowFunction)(LayoutUnit);
 
     virtual bool isOfType(LayoutObjectType type) const override { return type == LayoutObjectLayoutGrid || LayoutBlock::isOfType(type); }
     virtual void computeIntrinsicLogicalWidths(LayoutUnit& minLogicalWidth, LayoutUnit& maxLogicalWidth) const override;
     virtual void computePreferredLogicalWidths() 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 void styleDidChange(StyleDifference, const ComputedStyle*) override;
 
     bool explicitGridDidResize(const ComputedStyle&) const;
     bool namedGridLinesDefinitionDidChange(const ComputedStyle&) const;
 
     class GridIterator;
     struct GridSizingData;
     bool gridElementIsShrinkToFit();
@@ skipping 91 matching lines @@
     OrderIterator m_orderIterator;
     Vector<LayoutBox*> m_gridItemsOverflowingGridArea;
     HashMap<const LayoutBox*, size_t> m_gridItemsIndexesMap;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutGrid, isLayoutGrid());
 
 } // namespace blink
 
 #endif // LayoutGrid_h