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

Source/core/rendering/compositing/CompositedLayerMapping.cpp

Index: Source/core/rendering/compositing/CompositedLayerMapping.cpp
diff --git a/Source/core/rendering/compositing/CompositedLayerMapping.cpp b/Source/core/rendering/compositing/CompositedLayerMapping.cpp
index 143314471c68cc4a5bb08db571fa9608b2f04084..b9c5b7d7bc79d13d63bbd5e2c191e773d1acf9c8 100644
--- a/Source/core/rendering/compositing/CompositedLayerMapping.cpp
+++ b/Source/core/rendering/compositing/CompositedLayerMapping.cpp
@@ -541,8 +541,7 @@ static IntRect clipBox(RenderBox* renderer)
 
 static LayoutPoint computeOffsetFromCompositedAncestor(const RenderLayer* layer, const RenderLayer* compositedAncestor)
 {
-    LayoutPoint offset;
-    layer->convertToLayerCoords(compositedAncestor, offset);
+    LayoutPoint offset = layer->visualOffsetFromAncestor(compositedAncestor);
     if (compositedAncestor)
         offset.move(compositedAncestor->compositedLayerMapping()->owningLayer().subpixelAccumulation());
     return offset;