[New Multicolumn] Support for block direction margins on spanners.

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 128 matching lines @@
     // The "CSS actual" value of column-count. This includes overflowing columns, if any.
     unsigned actualColumnCount() const;
 
 protected:
     RenderMultiColumnSet(RenderFlowThread*);
 
 private:
     virtual void insertedIntoTree() override final;
     virtual void willBeRemovedFromTree() override final;
 
+    virtual bool isSelfCollapsingBlock() const override { return false; }
+
     virtual void computeLogicalHeight(LayoutUnit logicalHeight, LayoutUnit logicalTop, LogicalExtentComputedValues&) const override;
 
     virtual void paintObject(const PaintInfo&, const LayoutPoint& paintOffset) override;
 
     virtual void addOverflowFromChildren() override;
 
     virtual const char* renderName() const override;
 
     LayoutUnit calculateMaxColumnHeight() const;
     LayoutRect columnRectAt(unsigned index) const;
@@ skipping 71 matching lines @@
     // initial margin collapsing estimates were wrong.
     setLogicalBottomInFlowThread(offsetInFlowThread);
 }
 
 DEFINE_RENDER_OBJECT_TYPE_CASTS(RenderMultiColumnSet, isRenderMultiColumnSet());
 
 } // namespace blink
 
 #endif // RenderMultiColumnSet_h