[New Multicolumn] Add RenderMultiColumnSpannerSet.

Source/core/rendering/RenderMultiColumnSet.h

 /*
  * Copyright (C) 2012 Apple Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 32 matching lines @@
 // Being a "region", a column set has no children on its own, but is merely used to slice a portion
 // of the tall "single-column" flow thread into actual columns visually, to convert from flow thread
 // coordinates to visual ones. It is in charge of both positioning columns correctly relatively to
 // the parent multicol container, and to calculate the correct translation for each column's
 // contents, and to paint any rules between them. RenderMultiColumnSet objects are used for
 // painting, hit testing, and any other type of operation that requires mapping from flow thread
 // coordinates to visual coordinates.
 //
 // 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 };
 
     static RenderMultiColumnSet* createAnonymous(RenderFlowThread*, RenderStyle* parentStyle);
 
     virtual bool isOfType(RenderObjectType type) const override { return type == RenderObjectRenderMultiColumnSet || RenderRegion::isOfType(type); }
 
     virtual LayoutUnit pageLogicalWidth() const override final { return flowThread()->logicalWidth(); }
     virtual LayoutUnit pageLogicalHeight() const override final { return m_columnHeight; }
 
@@ skipping 24 matching lines @@
     void updateMinimumColumnHeight(LayoutUnit height) { m_minimumColumnHeight = std::max(height, m_minimumColumnHeight); }
     LayoutUnit minimumColumnHeight() const { return m_minimumColumnHeight; }
 
     // Add a content run, specified by its end position. A content run is appended at every
     // forced/explicit break and at the end of the column set. The content runs are used to
     // determine where implicit/soft breaks will occur, in order to calculate an initial column
     // height.
     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
     // shortage we find is the amount with which we will stretch the column height, if it turns out
     // after layout that the columns weren't tall enough.
     void recordSpaceShortage(LayoutUnit spaceShortage);
 
     // Reset previously calculated column height. Will mark for layout if needed.
     void resetColumnHeight();
 
@@ skipping 11 matching lines @@
     // page.
     LayoutUnit pageLogicalTopForOffset(LayoutUnit offset) const;
 
     void collectLayerFragments(LayerFragments&, const LayoutRect& layerBoundingBox, const LayoutRect& dirtyRect);
 
     LayoutUnit columnGap() const;
 
     // 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;
 
     virtual void computeLogicalHeight(LayoutUnit logicalHeight, LayoutUnit logicalTop, LogicalExtentComputedValues&) const override;
 
     virtual void paintObject(PaintInfo&, const LayoutPoint& paintOffset) override;
 
     virtual void addOverflowFromChildren() override;
 
     virtual const char* renderName() const override;
@@ skipping 58 matching lines @@
     };
     Vector<ContentRun, 1> m_contentRuns;
 };
 
 DEFINE_RENDER_OBJECT_TYPE_CASTS(RenderMultiColumnSet, isRenderMultiColumnSet());
 
 } // namespace blink
 
 #endif // RenderMultiColumnSet_h