Only update composited scrolling state when necessary

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..675fbc718bef1643c67778a5c9ce81ad96af06ce 100644
--- a/Source/WebCore/rendering/RenderLayer.h
+++ b/Source/WebCore/rendering/RenderLayer.h
@@ -497,6 +497,10 @@ public:
     // layer itself.
     RenderLayer* stackingContainer() const;
 
+    // Gets the enclosing stacking context for this layer, excluding this
+    // layer itself.
+    RenderLayer* stackingContext() const;
+
     // Gets the enclosing stacking container for this layer, possibly the layer
     // itself, if it is a stacking container.
     RenderLayer* enclosingStackingContainer() { return isStackingContainer() ? this : stackingContainer(); }
@@ -864,6 +868,8 @@ private:
     bool shouldRepaintAfterLayout() const;
 
     void updateSelfPaintingLayer();
+    void updateIsNormalFlowOnly();
+    void updateVisibilityAfterStyleChange(const RenderStyle* oldStyle);
     void updateStackingContextsAfterStyleChange(const RenderStyle* oldStyle);
 
     void updateScrollbarsAfterStyleChange(const RenderStyle* oldStyle);
@@ -1147,6 +1153,7 @@ 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;