Make OverlapMap a reference. We never actually pass null.

Source/core/rendering/compositing/RenderLayerCompositor.h

 /*
  * Copyright (C) 2009 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 265 matching lines @@
     void clearMappingForRenderLayerIncludingDescendants(RenderLayer*);
 
     void recursiveRepaintLayer(RenderLayer*);
 
     void addToOverlapMap(OverlapMap&, RenderLayer*, const IntRect& layerBounds);
 
     // Forces an update for all frames of frame tree recursively. Used only when the mainFrame compositor is ready to
     // finish all deferred work.
     static void finishCompositingUpdateForFrameTree(LocalFrame*);
 
-    void computeCompositingRequirements(RenderLayer* ancestorLayer, RenderLayer*, OverlapMap*, struct CompositingRecursionData&, bool& descendantHas3DTransform, Vector<RenderLayer*>& unclippedDescendants, IntRect& absoluteDecendantBoundingBox);
+    void computeCompositingRequirements(RenderLayer* ancestorLayer, RenderLayer*, OverlapMap&, struct CompositingRecursionData&, bool& descendantHas3DTransform, Vector<RenderLayer*>& unclippedDescendants, IntRect& absoluteDecendantBoundingBox);
 
     // Defines which RenderLayers will paint into which composited backings, by allocating and destroying CompositedLayerMappings as needed.
     void assignLayersToBackings(RenderLayer*, bool& layersChanged);
     void assignLayersToBackingsInternal(RenderLayer*, SquashingState&, bool& layersChanged, RenderLayer* clippingAncestor);
 
     // Allocates, sets up hierarchy, and sets appropriate properties for the GraphicsLayers that correspond to a given
     // composited RenderLayer. Does nothing if the given RenderLayer does not have a CompositedLayerMapping.
     void updateGraphicsLayersMappedToRenderLayer(RenderLayer*);
 
     // Recurses down the tree, updating layer geometry only.
@@ skipping 88 matching lines @@
     OwnPtr<GraphicsLayer> m_layerForScrollCorner;
 #if USE(RUBBER_BANDING)
     OwnPtr<GraphicsLayer> m_layerForOverhangShadow;
 #endif
 };
 
 
 } // namespace WebCore
 
 #endif // RenderLayerCompositor_h