Make mapLocalToContainer() work properly for content that follows a column spanner.

Source/core/layout/LayoutMultiColumnSet.cpp

 /*
  * 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 142 matching lines @@
                 // If we're followed by a spanner, we need to balance.
                 return true;
             }
         }
     }
     return !flowThread->columnHeightAvailable();
 }
 
 LayoutSize LayoutMultiColumnSet::flowThreadTranslationAtOffset(LayoutUnit blockOffset) const
 {
-    const MultiColumnFragmentainerGroup& row = fragmentainerGroupAtFlowThreadOffset(blockOffset);
-    return row.offsetFromColumnSet() + row.flowThreadTranslationAtOffset(blockOffset);
+    return fragmentainerGroupAtFlowThreadOffset(blockOffset).flowThreadTranslationAtOffset(blockOffset);
 }
 
 LayoutPoint LayoutMultiColumnSet::visualPointToFlowThreadPoint(const LayoutPoint& visualPoint) const
 {
     const MultiColumnFragmentainerGroup& row = fragmentainerGroupAtVisualPoint(visualPoint);
     return row.visualPointToFlowThreadPoint(visualPoint - row.offsetFromColumnSet());
 }
 
 void LayoutMultiColumnSet::updateMinimumColumnHeight(LayoutUnit offsetInFlowThread, LayoutUnit height)
 {
@@ skipping 151 matching lines @@
 
 LayoutRect LayoutMultiColumnSet::flowThreadPortionRect() const
 {
     LayoutRect portionRect(LayoutUnit(), logicalTopInFlowThread(), pageLogicalWidth(), logicalHeightInFlowThread());
     if (!isHorizontalWritingMode())
         return portionRect.transposedRect();
     return portionRect;
 }
 
 }