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

Source/core/rendering/RenderMultiColumnSet.h

Index: Source/core/rendering/RenderMultiColumnSet.h
diff --git a/Source/core/rendering/RenderMultiColumnSet.h b/Source/core/rendering/RenderMultiColumnSet.h
index 98a68bfa14b2d9323c551de085dfee272016efee..bd32b4e1aaddacf8bb47e2885bf2fb674646c8b7 100644
--- a/Source/core/rendering/RenderMultiColumnSet.h
+++ b/Source/core/rendering/RenderMultiColumnSet.h
@@ -70,6 +70,10 @@ public:
         m_computedColumnCount = count;
     }
 
+    // Find the column that contains the given block offset, and return the translation needed to
+    // get from flow thread coordinates to visual coordinates.
+    LayoutSize flowThreadTranslationAtOffset(LayoutUnit) const;
+
     LayoutUnit heightAdjustedForSetOffset(LayoutUnit height) const;
 
     void updateMinimumColumnHeight(LayoutUnit height) { m_minimumColumnHeight = std::max(height, m_minimumColumnHeight); }