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

Source/core/rendering/RenderBox.cpp

Index: Source/core/rendering/RenderBox.cpp
diff --git a/Source/core/rendering/RenderBox.cpp b/Source/core/rendering/RenderBox.cpp
index 2dd1e45b9e36818bf5cf48e1035339ef52db6c8f..eec10e9731143c6905cb68820ea0738b1d3e6153 100644
--- a/Source/core/rendering/RenderBox.cpp
+++ b/Source/core/rendering/RenderBox.cpp
@@ -128,15 +128,6 @@ LayerType RenderBox::layerTypeRequired() const
     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();