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

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 5a75a77fe1964fd72531e871fa797fb5feb1f540..263055e34947fc87a7c3125c1b29bcdbd928f925 100644
--- a/Source/core/rendering/RenderMultiColumnSet.h
+++ b/Source/core/rendering/RenderMultiColumnSet.h
@@ -146,6 +146,8 @@ 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;