[New Multicolumn] Create RenderMultiColumnSet during renderer creation, not during layout.

Source/core/rendering/RenderFlowThread.h

Index: Source/core/rendering/RenderFlowThread.h
diff --git a/Source/core/rendering/RenderFlowThread.h b/Source/core/rendering/RenderFlowThread.h
index c0d7b3b1021d3b3e8c87da915cd9f1c9d765034a..c9668079d658748fab9363fbeda6163552970339 100644
--- a/Source/core/rendering/RenderFlowThread.h
+++ b/Source/core/rendering/RenderFlowThread.h
@@ -67,7 +67,7 @@ public:
 
     virtual bool nodeAtPoint(const HitTestRequest&, HitTestResult&, const HitTestLocation& locationInContainer, const LayoutPoint& accumulatedOffset, HitTestAction) OVERRIDE FINAL;
 
-    virtual void addRegionToThread(RenderRegion*);
+    virtual void addRegionToThread(RenderRegion*) = 0;
     virtual void removeRegionFromThread(RenderRegion*);
     const RenderRegionList& renderRegionList() const { return m_regionList; }
 
@@ -91,11 +91,7 @@ public:
     virtual void setPageBreak(LayoutUnit /*offset*/, LayoutUnit /*spaceShortage*/) { }
     virtual void updateMinimumPageHeight(LayoutUnit /*offset*/, LayoutUnit /*minHeight*/) { }
 
-    enum RegionAutoGenerationPolicy {
-        AllowRegionAutoGeneration,
-        DisallowRegionAutoGeneration,
-    };
-    RenderRegion* regionAtBlockOffset(LayoutUnit, bool extendLastRegion = false, RegionAutoGenerationPolicy = AllowRegionAutoGeneration);
+    virtual RenderRegion* regionAtBlockOffset(LayoutUnit) const;
 
     bool regionsHaveUniformLogicalHeight() const { return m_regionsHaveUniformLogicalHeight; }
 
@@ -135,8 +131,6 @@ protected:
     void updateRegionsFlowThreadPortionRect();
     bool shouldRepaint(const LayoutRect&) const;
 
-    virtual void autoGenerateRegionsToBlockOffset(LayoutUnit) { }
-
     bool cachedOffsetFromLogicalTopOfFirstRegion(const RenderBox*, LayoutUnit&) const;
     void setOffsetFromLogicalTopOfFirstRegion(const RenderBox*, LayoutUnit);
     void clearOffsetFromLogicalTopOfFirstRegion(const RenderBox*);