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

Source/core/layout/compositing/GraphicsLayerUpdater.cpp

 /*
  * Copyright (C) 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2014 Google 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
@@ skipping 21 matching lines @@
 #include "core/layout/compositing/CompositedDeprecatedPaintLayerMapping.h"
 #include "core/layout/compositing/DeprecatedPaintLayerCompositor.h"
 #include "core/paint/DeprecatedPaintLayer.h"
 #include "platform/TraceEvent.h"
 
 namespace blink {
 
 class GraphicsLayerUpdater::UpdateContext {
 public:
     UpdateContext()
-        : m_compositingStackingContext(0)
-        , m_compositingAncestor(0)
+        : m_compositingStackingContext(nullptr)
+        , m_compositingAncestor(nullptr)
     {
     }
 
     UpdateContext(const UpdateContext& other, const DeprecatedPaintLayer& layer)
         : m_compositingStackingContext(other.m_compositingStackingContext)
         , m_compositingAncestor(other.compositingContainer(layer))
     {
         CompositingState compositingState = layer.compositingState();
         if (compositingState != NotComposited && compositingState != PaintsIntoGroupedBacking) {
             m_compositingAncestor = &layer;
@@ skipping 67 matching lines @@
     if (layer.hasCompositedDeprecatedPaintLayerMapping())
         layer.compositedDeprecatedPaintLayerMapping()->assertNeedsToUpdateGraphicsLayerBitsCleared();
 
     for (DeprecatedPaintLayer* child = layer.firstChild(); child; child = child->nextSibling())
         assertNeedsToUpdateGraphicsLayerBitsCleared(*child);
 }
 
 #endif
 
 } // namespace blink