[New Multicolumn] Handle flow thread overflow correctly in flipped writing modes.

Source/core/layout/LayoutFlowThread.h

 /*
  * Copyright (C) 2011 Adobe Systems Incorporated. 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.
@@ skipping 89 matching lines @@
     // FIXME: These 2 functions should return a LayoutMultiColumnSet.
     LayoutRegion* firstRegion() const;
     LayoutRegion* lastRegion() const;
 
     virtual bool addForcedRegionBreak(LayoutUnit, LayoutObject* breakChild, bool isBefore, LayoutUnit* offsetBreakAdjustment = 0) { return false; }
 
     virtual bool isPageLogicalHeightKnown() const { return true; }
     bool pageLogicalSizeChanged() const { return m_pageLogicalSizeChanged; }
 
     void collectLayerFragments(LayerFragments&, const LayoutRect& layerBoundingBox, const LayoutRect& dirtyRect);
+
+    // Return the visual bounding box based on the supplied flow-thread bounding box. Both
+    // rectangles are completely physical in terms of writing mode.
     LayoutRect fragmentsBoundingBox(const LayoutRect& layerBoundingBox) const;
 
     LayoutPoint flowThreadPointToVisualPoint(const LayoutPoint& flowThreadPoint) const
     {
         return flowThreadPoint + columnOffset(flowThreadPoint);
     }
 
     // Used to estimate the maximum height of the flow thread.
     static LayoutUnit maxLogicalHeight() { return LayoutUnit::max() / 2; }
 
@@ skipping 44 matching lines @@
 };
 
 template <> struct ValueToString<LayoutMultiColumnSet*> {
     static String string(const LayoutMultiColumnSet* value) { return String::format("%p", value); }
 };
 #endif
 
 } // namespace blink
 
 #endif // LayoutFlowThread_h