[New Multicolumn] flowThreadTranslationAtOffset() needs to be relative to the flow thread.

Source/core/layout/MultiColumnFragmentainerGroup.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MultiColumnFragmentainerGroup_h
 #define MultiColumnFragmentainerGroup_h
 
 #include "core/layout/LayoutMultiColumnFlowThread.h"
 
 namespace blink {
@@ skipping 65 matching lines @@
     // breaks assumed so far into account.
     unsigned findRunWithTallestColumns() const;
 
     // Given the current list of content runs, make assumptions about where we need to insert
     // implicit breaks (if there's room for any at all; depending on the number of explicit breaks),
     // and store the results. This is needed in order to balance the columns.
     void distributeImplicitBreaks();
 
     LayoutUnit calculateColumnHeight(BalancedColumnHeightCalculation) const;
 
+    LayoutSize translationAtColumn(unsigned columnIndex) const;
     LayoutRect columnRectAt(unsigned columnIndex) const;
     LayoutUnit logicalTopInFlowThreadAt(unsigned columnIndex) const { return m_logicalTopInFlowThread + columnIndex * m_columnHeight; }
     LayoutRect flowThreadPortionRectAt(unsigned columnIndex) const;
     LayoutRect flowThreadPortionOverflowRect(const LayoutRect& flowThreadPortion, unsigned columnIndex, unsigned columnCount, LayoutUnit columnGap) const;
 
     enum ColumnIndexCalculationMode {
         ClampToExistingColumns, // Stay within the range of already existing columns.
         AssumeNewColumns // Allow column indices outside the range of already existing columns.
     };
     unsigned columnIndexAtOffset(LayoutUnit offsetInFlowThread, ColumnIndexCalculationMode = ClampToExistingColumns) const;
@@ skipping 76 matching lines @@
 
 private:
     LayoutMultiColumnSet& m_columnSet;
 
     Vector<MultiColumnFragmentainerGroup, 1> m_groups;
 };
 
 } // namespace blink
 
 #endif // MultiColumnFragmentainerGroup_h