[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 deaa5e336fe963caa2cd03ff9d7c9f47076e2267..a1657e895d954f26468a912e6e267cc5da334075 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; }
 
@@ -94,11 +94,7 @@ 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);
 
     bool regionsHaveUniformLogicalHeight() const { return m_regionsHaveUniformLogicalHeight; }
 
@@ -141,8 +137,6 @@ 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*);