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

Source/core/rendering/RenderBox.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. 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
@@ skipping 110 matching lines @@
             return NoLayer;
         }
         return NormalLayer;
     }
     if (hasOverflowClip())
         return OverflowClipLayer;
 
     return NoLayer;
 }
 
-void RenderBox::willBeRemovedFromTree()
-{
-    if (m_rareData && m_rareData->m_spannerPlaceholder) {
-        m_rareData->m_spannerPlaceholder->spannerWillBeRemoved();
-        m_rareData->m_spannerPlaceholder = 0;
-    }
-    RenderBoxModelObject::willBeRemovedFromTree();
-}
-
 void RenderBox::willBeDestroyed()
 {
     clearOverrideSize();
     clearContainingBlockOverrideSize();
     clearExtraInlineAndBlockOffests();
 
     RenderBlock::removePercentHeightDescendantIfNeeded(this);
 
     ShapeOutsideInfo::removeInfo(*this);
 
@@ skipping 4410 matching lines @@
     computedValues.m_margins.m_end = marginEnd();
 
     setLogicalTop(oldLogicalTop);
     setLogicalWidth(oldLogicalWidth);
     setLogicalLeft(oldLogicalLeft);
     setMarginLeft(oldMarginLeft);
     setMarginRight(oldMarginRight);
 }
 
 } // namespace blink