Region based multicol: support explicit column breaks

Source/core/rendering/RenderFlowThread.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 128 matching lines @@
 
     void clearRenderObjectCustomStyle(const RenderObject*,
         const RenderRegion* oldStartRegion = 0, const RenderRegion* oldEndRegion = 0,
         const RenderRegion* newStartRegion = 0, const RenderRegion* newEndRegion = 0);
 
     // Check if the object is in region and the region is part of this flow thread.
     bool objectInFlowRegion(const RenderObject*, const RenderRegion*) const;
 
     void markAutoLogicalHeightRegionsForLayout();
 
-    bool addForcedRegionBreak(LayoutUnit, RenderObject* breakChild, bool isBefore, LayoutUnit* offsetBreakAdjustment = 0);
+    virtual bool addForcedRegionBreak(LayoutUnit, RenderObject* breakChild, bool isBefore, LayoutUnit* offsetBreakAdjustment = 0);
     void applyBreakAfterContent(LayoutUnit);
 
     bool pageLogicalSizeChanged() const { return m_pageLogicalSizeChanged; }
 
     bool hasAutoLogicalHeightRegions() const { ASSERT(isAutoLogicalHeightRegionsCountConsistent()); return m_autoLogicalHeightRegionsCount; }
     void incrementAutoLogicalHeightRegions();
     void decrementAutoLogicalHeightRegions();
 
 #ifndef NDEBUG
     bool isAutoLogicalHeightRegionsCountConsistent() const;
@@ skipping 154 matching lines @@
 };
 
 template <> struct ValueToString<RenderRegion*> {
     static String string(const RenderRegion* value) { return String::format("%p", value); }
 };
 #endif
 
 } // namespace WebCore
 
 #endif // RenderFlowThread_h