Region based multicol: support explicit column breaks

Source/core/rendering/RenderMultiColumnBlock.h

 /*
  * 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 55 matching lines @@
     void computeColumnCountAndWidth();
 
     void ensureColumnSets();
 
     RenderMultiColumnFlowThread* m_flowThread;
     unsigned m_columnCount;   // The default column count/width that are based off our containing block width. These values represent only the default,
     LayoutUnit m_columnWidth; // since a multi-column block that is split across variable width pages or regions will have different column counts and widths in each.
                               // These values will be cached (eventually) for multi-column blocks.
     LayoutUnit m_columnHeightAvailable; // Total height available to columns, or 0 if auto.
     bool m_inBalancingPass; // Set when relayouting for column balancing.
+    bool m_needsRebalancing;
 };
 
 DEFINE_RENDER_OBJECT_TYPE_CASTS(RenderMultiColumnBlock, isRenderMultiColumnBlock());
 
 } // namespace WebCore
 
 #endif // RenderMultiColumnBlock_h