Avoid tree walks when computing RenderLayer::scrollParent

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 cf9654aa369d9899df8cf2db658482a291ef91a7..abd5115e989406e9245f185b6a0ec9ecb4b6bcf5 100644
--- a/Source/core/rendering/RenderLayer.h
+++ b/Source/core/rendering/RenderLayer.h
@@ -358,6 +358,9 @@ public:
     // compositing state may legally be read.
     bool isAllowedToQueryCompositingState() const;
 
+    // This returns true if our current phase is the compositing update.
+    bool isInCompositingUpdate() const;
+
     CompositedLayerMappingPtr compositedLayerMapping() const;
     CompositedLayerMappingPtr ensureCompositedLayerMapping();
 
@@ -479,7 +482,18 @@ public:
 
     bool hasDirectReasonsForCompositing() const { return compositingReasons() & CompositingReasonComboAllDirectReasons; }
 
+    void clearAncestorDependentPropertyCache();
+
 private:
+    struct AncestorDependentPropertyCache {
+        AncestorDependentPropertyCache();
+        RenderObject* containingBlock;
+        RenderLayer* ancestorCompositedScrollingLayer;
+        RenderLayer* scrollParent;
+    };
+
+    void ensureAncestorDependentPropertyCache() const;
+
     bool hasOverflowControls() const;
 
     void setIsUnclippedDescendant(bool isUnclippedDescendant) { m_isUnclippedDescendant = isUnclippedDescendant; }
@@ -578,7 +592,6 @@ private:
 
     bool shouldBeSelfPaintingLayer() const;
 
-private:
     // FIXME: We should only create the stacking node if needed.
     bool requiresStackingNode() const { return true; }
     void updateStackingNode();
@@ -630,7 +643,6 @@ private:
     friend class RenderLayerCompositor;
     friend class RenderLayerModelObject;
 
-private:
     LayerType m_layerType;
 
     // Self-painting layer is an optimization where we avoid the heavy RenderLayer painting
@@ -756,6 +768,8 @@ private:
     OwnPtr<CompositedLayerMapping> m_compositedLayerMapping;
     OwnPtr<RenderLayerScrollableArea> m_scrollableArea;
 
+    mutable OwnPtr<AncestorDependentPropertyCache> m_ancestorDependentPropertyCache;
+
     CompositedLayerMapping* m_groupedMapping;
 
     RenderLayerRepainter m_repainter;