[New Multicolumn] mapAbsoluteToLocalPoint() needs to convert to flow thread coordinates.

Source/core/layout/LayoutFlowThread.h

Index: Source/core/layout/LayoutFlowThread.h
diff --git a/Source/core/layout/LayoutFlowThread.h b/Source/core/layout/LayoutFlowThread.h
index 88face64a7f5d57682c713a456a18af30a48605e..d9c3a1d1ce08290b94600402420c79153df152e5 100644
--- a/Source/core/layout/LayoutFlowThread.h
+++ b/Source/core/layout/LayoutFlowThread.h
@@ -117,6 +117,8 @@ public:
         return flowThreadPoint + columnOffset(flowThreadPoint);
     }
 
+    virtual LayoutPoint visualPointToFlowThreadPoint(const LayoutPoint& visualPoint) const = 0;
+
     // Used to estimate the maximum height of the flow thread.
     static LayoutUnit maxLogicalHeight() { return LayoutUnit::max() / 2; }