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

Source/core/layout/LayoutGrid.cpp

 /*
  * 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 152 matching lines @@
 
         size_t& varyingTrackIndex = (m_direction == ForColumns) ? m_rowIndex : m_columnIndex;
         const size_t endOfVaryingTrackIndex = (m_direction == ForColumns) ? m_grid.size() : m_grid[0].size();
         for (; varyingTrackIndex < endOfVaryingTrackIndex; ++varyingTrackIndex) {
             const GridCell& children = m_grid[m_rowIndex][m_columnIndex];
             if (m_childIndex < children.size())
                 return children[m_childIndex++];
 
             m_childIndex = 0;
         }
-        return 0;
+        return nullptr;
     }
 
     bool checkEmptyCells(size_t rowSpan, size_t columnSpan) const
     {
         // Ignore cells outside current grid as we will grow it later if needed.
         size_t maxRows = std::min(m_rowIndex + rowSpan, m_grid.size());
         size_t maxColumns = std::min(m_columnIndex + columnSpan, m_grid[0].size());
 
         // This adds a O(N^2) behavior that shouldn't be a big deal as we expect spanning areas to be small.
         for (size_t row = m_rowIndex; row < maxRows; ++row) {
@@ skipping 1645 matching lines @@
 
     return LayoutPoint(columnPosition, rowPositionForChild(child));
 }
 
 void LayoutGrid::paintChildren(const PaintInfo& paintInfo, const LayoutPoint& paintOffset)
 {
     GridPainter(*this).paintChildren(paintInfo, paintOffset);
 }
 
 } // namespace blink