[New Multicolumn] Make computeOffsetFromCompositedAncestor() flowthread-aware.

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 61823f49fcc5a47b661b7abf228d8a8f9096df32..9c8e84886ffed85c0ada141630e03023e528798b 100644
--- a/Source/core/rendering/RenderLayer.h
+++ b/Source/core/rendering/RenderLayer.h
@@ -215,6 +215,8 @@ public:
     void convertToLayerCoords(const RenderLayer* ancestorLayer, LayoutPoint&) const;
     void convertToLayerCoords(const RenderLayer* ancestorLayer, LayoutRect&) const;
 
+    LayoutPoint visualOffsetFromAncestor(const RenderLayer* ancestorLayer) const;
+
     // The hitTest() method looks for mouse events by walking layers that intersect the point from front to back.
     bool hitTest(const HitTestRequest&, HitTestResult&);
     bool hitTest(const HitTestRequest&, const HitTestLocation&, HitTestResult&);