Shorten flowThreadDescendantWasInserted() and flowThreadDescendantWillBeRemoved().

Source/core/layout/LayoutMultiColumnFlowThread.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 171 matching lines @@
 
 protected:
     LayoutMultiColumnFlowThread();
     void setProgressionIsInline(bool isInline) { m_progressionIsInline = isInline; }
 
     virtual void layout() override;
 
 private:
     void calculateColumnCountAndWidth(LayoutUnit& width, unsigned& count) const;
     void createAndInsertMultiColumnSet(LayoutBox* insertBefore = nullptr);
-    void createAndInsertSpannerPlaceholder(LayoutBox* spanner, LayoutBox* insertBefore = nullptr);
+    void createAndInsertSpannerPlaceholder(LayoutBox* spannerObjectInFlowThread, LayoutObject* insertedBeforeInFlowThread);
+    void destroySpannerPlaceholder(LayoutMultiColumnSpannerPlaceholder*);
     virtual bool descendantIsValidColumnSpanner(LayoutObject* descendant) const;
 
     virtual void addColumnSetToThread(LayoutMultiColumnSet*) override;
     virtual void willBeRemovedFromTree() override;
     virtual LayoutUnit skipColumnSpanner(LayoutBox*, LayoutUnit logicalTopInFlowThread) override;
     virtual void flowThreadDescendantWasInserted(LayoutObject*) override;
     virtual void flowThreadDescendantWillBeRemoved(LayoutObject*) override;
     virtual void flowThreadDescendantStyleWillChange(LayoutObject*, StyleDifference, const ComputedStyle& newStyle) override;
     virtual void flowThreadDescendantStyleDidChange(LayoutObject*, StyleDifference, const ComputedStyle& oldStyle) override;
     virtual void computePreferredLogicalWidths() override;
@@ skipping 13 matching lines @@
     LayoutUnit m_columnHeightAvailable; // Total height available to columns, or 0 if auto.
     bool m_inBalancingPass; // Set when relayouting for column balancing.
     bool m_needsColumnHeightsRecalculation; // Set when we need to recalculate the column set heights after layout.
     bool m_progressionIsInline; // Always true for regular multicol. False for paged-y overflow.
     bool m_isBeingEvacuated;
 };
 
 } // namespace blink
 
 #endif // LayoutMultiColumnFlowThread_h