[New Multicolumn] Make offsetLeft, offsetTop and getClientRects() behave.

Source/core/rendering/RenderMultiColumnFlowThread.h

Index: Source/core/rendering/RenderMultiColumnFlowThread.h
diff --git a/Source/core/rendering/RenderMultiColumnFlowThread.h b/Source/core/rendering/RenderMultiColumnFlowThread.h
index 639c9b513baa4bf2a5a1a6197027c7b7e714fff9..c0c7e6f355b60e793da36e6f15296586d1754164 100644
--- a/Source/core/rendering/RenderMultiColumnFlowThread.h
+++ b/Source/core/rendering/RenderMultiColumnFlowThread.h
@@ -78,6 +78,8 @@ public:
     void setColumnHeightAvailable(LayoutUnit available) { m_columnHeightAvailable = available; }
     bool requiresBalancing() const { return !columnHeightAvailable() || multiColumnBlockFlow()->style()->columnFill() == ColumnFillBalance; }
 
+    virtual LayoutSize columnOffset(const LayoutPoint&) const OVERRIDE FINAL;
+
     void layoutColumns(bool relayoutChildren, SubtreeLayoutScope&);
     bool computeColumnCountAndWidth();
     bool recalculateColumnHeights();