Clean up GraphicsLayerPaintInfo to remove unused or unnecessary fields.

Source/core/rendering/RenderObject.h

Index: Source/core/rendering/RenderObject.h
diff --git a/Source/core/rendering/RenderObject.h b/Source/core/rendering/RenderObject.h
index f9485675c11fdb46810c8ec0689d0a86ed53d01c..6b76ee763b39cad4b3f4d7c634b19bb79d93eb50 100644
--- a/Source/core/rendering/RenderObject.h
+++ b/Source/core/rendering/RenderObject.h
@@ -841,7 +841,7 @@ public:
     // Return the RenderLayerModelObject in the container chain which is responsible for painting this object, or 0
     // if painting is root-relative. This is the container that should be passed to the 'forPaintInvalidation'
     // methods.
-    const RenderLayerModelObject* containerForPaintInvalidation() const;
+    virtual const RenderLayerModelObject* containerForPaintInvalidation() const;
     const RenderLayerModelObject* adjustCompositedContainerForSpecialAncestors(const RenderLayerModelObject* paintInvalidationContainer) const;
     bool isPaintInvalidationContainer() const;