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

Source/core/layout/compositing/CompositingLayerAssigner.h

 /*
  * 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 34 matching lines @@
 
     bool layersChanged() const { return m_layersChanged; }
 
     // FIXME: This function should be private. We should remove the one caller
     // once we've fixed the compositing chicken/egg issues.
     CompositingStateTransitionType computeCompositedLayerUpdate(DeprecatedPaintLayer*);
 
 private:
     struct SquashingState {
         SquashingState()
-            : mostRecentMapping(0)
+            : mostRecentMapping(nullptr)
             , hasMostRecentMapping(false)
             , haveAssignedBackingsToEntireSquashingLayerSubtree(false)
             , nextSquashedLayerIndex(0)
             , totalAreaOfSquashedRects(0) { }
 
         void updateSquashingStateForNewMapping(CompositedDeprecatedPaintLayerMapping*, bool hasNewCompositedDeprecatedPaintLayerMapping);
 
         // The most recent composited backing that the layer should squash onto if needed.
         CompositedDeprecatedPaintLayerMapping* mostRecentMapping;
         bool hasMostRecentMapping;
@@ skipping 21 matching lines @@
     void updateSquashingAssignment(DeprecatedPaintLayer*, SquashingState&, CompositingStateTransitionType, Vector<DeprecatedPaintLayer*>& layersNeedingPaintInvalidation);
     bool needsOwnBacking(const DeprecatedPaintLayer*) const;
 
     DeprecatedPaintLayerCompositor* m_compositor;
     bool m_layersChanged;
 };
 
 } // namespace blink
 
 #endif // CompositingLayerAssigner_h