[New Multicolumn] Remove RenderRegionSet.

Source/core/rendering/RenderMultiColumnSet.h

Index: Source/core/rendering/RenderMultiColumnSet.h
diff --git a/Source/core/rendering/RenderMultiColumnSet.h b/Source/core/rendering/RenderMultiColumnSet.h
index 57b09aa1136649e8c7f6369f2307dd737150b68e..97c314207f955fb12a1536276a59892f83e20f31 100644
--- a/Source/core/rendering/RenderMultiColumnSet.h
+++ b/Source/core/rendering/RenderMultiColumnSet.h
@@ -28,7 +28,7 @@
 #define RenderMultiColumnSet_h
 
 #include "core/rendering/RenderMultiColumnFlowThread.h"
-#include "core/rendering/RenderRegionSet.h"
+#include "core/rendering/RenderRegion.h"
 #include "wtf/Vector.h"
 
 namespace WebCore {
@@ -50,7 +50,7 @@ namespace WebCore {
 //
 // Column spans result in the creation of new column sets, since a spanning renderer has to be
 // placed in between the column sets that come before and after the span.
-class RenderMultiColumnSet FINAL : public RenderRegionSet {
+class RenderMultiColumnSet FINAL : public RenderRegion {
 public:
     static RenderMultiColumnSet* createAnonymous(RenderFlowThread*, RenderStyle* parentStyle);
 
@@ -103,6 +103,10 @@ public:
 
     void prepareForLayout();
 
+    // Expand this set's flow thread portion rectangle to contain all trailing flow thread
+    // overflow. Only to be called on the last set.
+    void expandToEncompassFlowThreadContentsIfNeeded();
+
 private:
     RenderMultiColumnSet(RenderFlowThread*);