[New Multicolumn] Add RenderMultiColumnSpannerSet.

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 3bf8b167807e0add8cdba42dd6a1e2bbe8c068af..a0ca2c41500b22d1a26632194d2e227d3ffae9ae 100644
--- a/Source/core/rendering/RenderMultiColumnSet.h
+++ b/Source/core/rendering/RenderMultiColumnSet.h
@@ -50,7 +50,7 @@ namespace blink {
 //
 // 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 RenderRegion {
+class RenderMultiColumnSet : public RenderRegion {
 public:
     enum BalancedHeightCalculation { GuessFromFlowThreadPortion, StretchBySpaceShortage };
 
@@ -95,7 +95,7 @@ public:
     void addContentRun(LayoutUnit endOffsetFromFirstPage);
 
     // (Re-)calculate the column height if it's auto.
-    bool recalculateColumnHeight(BalancedHeightCalculation);
+    virtual bool recalculateColumnHeight(BalancedHeightCalculation);
 
     // Record space shortage (the amount of space that would have been enough to prevent some
     // element from being moved to the next column) at a column break. The smallest amount of space
@@ -127,9 +127,10 @@ public:
     // The "CSS actual" value of column-count. This includes overflowing columns, if any.
     unsigned actualColumnCount() const;
 
-private:
+protected:
     RenderMultiColumnSet(RenderFlowThread*);
 
+private:
     virtual void insertedIntoTree() override final;
     virtual void willBeRemovedFromTree() override final;