[New Multicolumn] Support dynamic insertion and removal of multicol content.

Source/core/rendering/RenderBox.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2006, 2007 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 633 matching lines @@
     void markShapeOutsideDependentsForLayout()
     {
         if (isFloating())
             removeFloatingOrPositionedChildFromBlockLists();
     }
 
     bool backgroundHasOpaqueTopLayer() const;
 
     void setIntrinsicContentLogicalHeight(LayoutUnit intrinsicContentLogicalHeight) const { m_intrinsicContentLogicalHeight = intrinsicContentLogicalHeight; }
 protected:
-    virtual void willBeRemovedFromTree() override;
     virtual void willBeDestroyed() override;
 
     virtual void styleWillChange(StyleDifference, const RenderStyle& newStyle) override;
     virtual void styleDidChange(StyleDifference, const RenderStyle* oldStyle) override;
     virtual void updateFromStyle() override;
 
     // Returns false if it could not cheaply compute the extent (e.g. fixed background), in which case the returned rect may be incorrect.
     // FIXME: make this a const method once the RenderBox reference in BoxPainter is const.
     bool getBackgroundPaintedExtent(LayoutRect&);
     virtual bool foregroundIsKnownToBeOpaqueInRect(const LayoutRect& localRect, unsigned maxDepthToTest) const;
@@ skipping 177 matching lines @@
         if (UNLIKELY(inlineBoxWrapper() != 0))
             deleteLineBoxWrapper();
     }
 
     ensureRareData().m_inlineBoxWrapper = boxWrapper;
 }
 
 } // namespace blink
 
 #endif // RenderBox_h