Add methods to RenderLayer to get at the painting or scrolling graphics layers, if present.

Source/core/rendering/RenderLayer.h

Index: Source/core/rendering/RenderLayer.h
diff --git a/Source/core/rendering/RenderLayer.h b/Source/core/rendering/RenderLayer.h
index c7a3efdb424a2ce8923ab4abfa559cb1971370a6..9b30d0ca6e31e1a23c949569f43648ff3592850d 100644
--- a/Source/core/rendering/RenderLayer.h
+++ b/Source/core/rendering/RenderLayer.h
@@ -313,15 +313,16 @@ public:
     // compositing state may legally be read.
     bool isAllowedToQueryCompositingState() const;
 
+    // NOTE: Don't call these compositing methods unless you know what you are doing and are sure it is the best approach!
     CompositedLayerMappingPtr compositedLayerMapping() const;
     CompositedLayerMappingPtr ensureCompositedLayerMapping();
-
+    GraphicsLayer* graphicsLayerBacking() const;
+    GraphicsLayer* graphicsLayerBackingForScrolling() const;
     // NOTE: If you are using hasCompositedLayerMapping to determine the state of compositing for this layer,
     // (and not just to do bookkeeping related to the mapping like, say, allocating or deallocating a mapping),
     // then you may have incorrect logic. Use compositingState() instead.
     bool hasCompositedLayerMapping() const { return m_compositedLayerMapping.get(); }
     void clearCompositedLayerMapping(bool layerBeingDestroyed = false);
-
     CompositedLayerMapping* groupedMapping() const { return m_groupedMapping; }
     void setGroupedMapping(CompositedLayerMapping* groupedMapping, bool layerBeingDestroyed = false);