Rework composited scrolling updates.

Source/WebCore/rendering/RenderLayer.h

Index: Source/WebCore/rendering/RenderLayer.h
diff --git a/Source/WebCore/rendering/RenderLayer.h b/Source/WebCore/rendering/RenderLayer.h
index e5fc1d71d9c3cc0d11508317eb7736991ae687af..0a6a419accde4e6d6594975595331067eda58c75 100644
--- a/Source/WebCore/rendering/RenderLayer.h
+++ b/Source/WebCore/rendering/RenderLayer.h
@@ -864,6 +864,7 @@ private:
     bool shouldRepaintAfterLayout() const;
 
     void updateSelfPaintingLayer();
+    void updateIsNormalFlowOnly();
     void updateStackingContextsAfterStyleChange(const RenderStyle* oldStyle);
 
     void updateScrollbarsAfterStyleChange(const RenderStyle* oldStyle);
@@ -874,6 +875,7 @@ private:
     void updateOutOfFlowPositioned(const RenderStyle* oldStyle);
 
     void updateNeedsCompositedScrolling();
+    void setNeedsCompositedScrolling(bool);
 
     // Returns true if the position changed.
     bool updateLayerPosition();
@@ -1147,6 +1149,7 @@ protected:
     // descendants in stacking order. This is one of the requirements of being
     // able to safely become a stacking context.
     bool m_descendantsAreContiguousInStackingOrder : 1;
+    bool m_descendantsAreContiguousInStackingOrderDirty : 1;
 
     const bool m_isRootLayer : 1;