[New Multicolumn] Preparatory work for nested multicol support.

Source/core/rendering/RenderRegion.h

Index: Source/core/rendering/RenderRegion.h
diff --git a/Source/core/rendering/RenderRegion.h b/Source/core/rendering/RenderRegion.h
index fbd3711f363a5ee7886ae8a807afb7859ea7a8a0..a48ed5ffd8138e02f3a771cdc68dfee8984b7e7c 100644
--- a/Source/core/rendering/RenderRegion.h
+++ b/Source/core/rendering/RenderRegion.h
@@ -42,9 +42,9 @@ public:
 
     virtual bool isOfType(LayoutObjectType type) const override { return type == LayoutObjectRenderRegion || RenderBlockFlow::isOfType(type); }
 
-    void setFlowThreadPortionRect(const LayoutRect& rect) { m_flowThreadPortionRect = rect; }
-    LayoutRect flowThreadPortionRect() const { return m_flowThreadPortionRect; }
+    virtual LayoutRect flowThreadPortionRect() const = 0;
     LayoutRect flowThreadPortionOverflowRect() const;
+    LayoutRect overflowRectForFlowThreadPortion(const LayoutRect& flowThreadPortionRect, bool isFirstPortion, bool isLastPortion) const;
 
     RenderFlowThread* flowThread() const { return m_flowThread; }
 
@@ -69,8 +69,6 @@ public:
 protected:
     virtual void computeIntrinsicLogicalWidths(LayoutUnit& minLogicalWidth, LayoutUnit& maxLogicalWidth) const override final;
 
-    LayoutRect overflowRectForFlowThreadPortion(const LayoutRect& flowThreadPortionRect, bool isFirstPortion, bool isLastPortion) const;
-
 private:
     virtual void layoutBlock(bool relayoutChildren) override final;
 
@@ -78,7 +76,6 @@ protected:
     RenderFlowThread* m_flowThread;
 
 private:
-    LayoutRect m_flowThreadPortionRect;
     bool m_isValid : 1;
 };