[New Multicolumn] Remove RenderRegionSet.

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 66f39852d40af63af603c5c2dd51cd322d49baa8..c25cebb166e5c16219d5ffe900bf42714d3ce9c5 100644
--- a/Source/core/rendering/RenderRegion.h
+++ b/Source/core/rendering/RenderRegion.h
@@ -62,9 +62,9 @@ public:
     bool isFirstRegion() const;
     bool isLastRegion() const;
 
-    // These methods represent the width and height of a "page" and for a RenderRegion they are just the
-    // content width and content height of a region. For RenderRegionSets, however, they will be the width and
-    // height of a single column or page in the set.
+    // These methods represent the width and height of a "page" and for a RenderRegion they are just
+    // the content width and content height of a region. For RenderMultiColumnSets, however, they
+    // will be the width and height of a single column or page in the set.
     virtual LayoutUnit pageLogicalWidth() const;
     virtual LayoutUnit pageLogicalHeight() const;
 
@@ -83,8 +83,6 @@ public:
     // page.
     virtual LayoutUnit pageLogicalTopForOffset(LayoutUnit offset) const;
 
-    virtual void expandToEncompassFlowThreadContentsIfNeeded() { };
-
     virtual void repaintFlowThreadContent(const LayoutRect& repaintRect) const;
 
     virtual void collectLayerFragments(LayerFragments&, const LayoutRect&, const LayoutRect&) { }