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

Source/core/rendering/compositing/CompositedLayerMapping.cpp

 /*
  * Copyright (C) 2009, 2010, 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 523 matching lines @@
         result = renderer->overflowClipRect(LayoutPoint());
 
     if (renderer->hasClip())
         result.intersect(renderer->clipRect(LayoutPoint()));
 
     return pixelSnappedIntRect(result);
 }
 
 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;
 }
 
 void CompositedLayerMapping::computeBoundsOfOwningLayer(const RenderLayer* compositedAncestor, IntRect& localBounds, IntRect& compositingBoundsRelativeToCompositedAncestor, LayoutPoint& offsetFromCompositedAncestor,
     IntPoint& snappedOffsetFromCompositedAncestor)
 {
     LayoutRect localRawCompositingBounds = compositedBounds();
     offsetFromCompositedAncestor = computeOffsetFromCompositedAncestor(&m_owningLayer, compositedAncestor);
@@ skipping 1762 matching lines @@
     } else if (graphicsLayer == m_scrollingBlockSelectionLayer.get()) {
         name = "Scrolling Block Selection Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace blink