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

Source/core/rendering/RenderMultiColumnFlowThread.h

 /*
  * Copyright (C) 2012 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 60 matching lines @@
     // renderers. Then destroy the flow thread. Called when a multicol container becomes a regular
     // block.
     void evacuateAndDestroy();
 
     unsigned columnCount() const { return m_columnCount; }
     LayoutUnit columnWidth() const { return m_columnWidth; }
     LayoutUnit columnHeightAvailable() const { return m_columnHeightAvailable; }
     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();
 
 private:
     RenderMultiColumnFlowThread();
 
     virtual const char* renderName() const OVERRIDE;
     virtual void addRegionToThread(RenderRegion*) OVERRIDE;
     virtual void willBeRemovedFromTree() OVERRIDE;
     virtual void computeLogicalHeight(LayoutUnit logicalHeight, LayoutUnit logicalTop, LogicalExtentComputedValues&) const OVERRIDE;
     virtual LayoutUnit initialLogicalWidth() const OVERRIDE;
     virtual void setPageBreak(LayoutUnit offset, LayoutUnit spaceShortage) OVERRIDE;
     virtual void updateMinimumPageHeight(LayoutUnit offset, LayoutUnit minHeight) OVERRIDE;
     virtual RenderRegion* regionAtBlockOffset(LayoutUnit) const OVERRIDE;
     virtual bool addForcedRegionBreak(LayoutUnit, RenderObject* breakChild, bool isBefore, LayoutUnit* offsetBreakAdjustment = 0) OVERRIDE;
     virtual bool isPageLogicalHeightKnown() const OVERRIDE;
 
     unsigned m_columnCount; // The used value of column-count
     LayoutUnit m_columnWidth; // The used value of column-width
     LayoutUnit m_columnHeightAvailable; // Total height available to columns, or 0 if auto.
     bool m_inBalancingPass; // Set when relayouting for column balancing.
     bool m_needsRebalancing;
 };
 
 } // namespace WebCore
 
 #endif // RenderMultiColumnFlowThread_h