Region based multicol: support explicit column breaks

Source/core/rendering/RenderMultiColumnBlock.h

Index: Source/core/rendering/RenderMultiColumnBlock.h
diff --git a/Source/core/rendering/RenderMultiColumnBlock.h b/Source/core/rendering/RenderMultiColumnBlock.h
index 0e6aed38536a1a3deeaedddbe033ede9e4a8b63b..8b6115e08dae600cf979f4a83cc9a89bcc088252 100644
--- a/Source/core/rendering/RenderMultiColumnBlock.h
+++ b/Source/core/rendering/RenderMultiColumnBlock.h
@@ -46,7 +46,7 @@ public:
 
     bool requiresBalancing() const { return !m_columnHeightAvailable || style()->columnFill() == ColumnFillBalance; }
 
-    bool shouldRelayoutMultiColumnBlock() const;
+    bool shouldRelayoutMultiColumnBlock();
 
 private:
     virtual bool isRenderMultiColumnBlock() const OVERRIDE { return true; }
@@ -73,7 +73,8 @@ private:
     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.
-    mutable bool m_inBalancingPass; // Set when relayouting for column balancing.
+    bool m_inBalancingPass; // Set when relayouting for column balancing.
+    bool m_needsRebalancing;
 };
 
 DEFINE_RENDER_OBJECT_TYPE_CASTS(RenderMultiColumnBlock, isRenderMultiColumnBlock());