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

Source/core/rendering/RenderMultiColumnSpannerPlaceholder.cpp

Index: Source/core/rendering/RenderMultiColumnSpannerPlaceholder.cpp
diff --git a/Source/core/rendering/RenderMultiColumnSpannerPlaceholder.cpp b/Source/core/rendering/RenderMultiColumnSpannerPlaceholder.cpp
index a4ce37927b0abdb3624c3ac6cbe379e69bf0b40c..ddf778817f44548ba829445e84840bf5ec7b3b44 100644
--- a/Source/core/rendering/RenderMultiColumnSpannerPlaceholder.cpp
+++ b/Source/core/rendering/RenderMultiColumnSpannerPlaceholder.cpp
@@ -34,15 +34,6 @@ RenderMultiColumnSpannerPlaceholder::RenderMultiColumnSpannerPlaceholder(RenderB
 {
 }
 
-void RenderMultiColumnSpannerPlaceholder::spannerWillBeRemoved()
-{
-    ASSERT(m_rendererInFlowThread);
-    RenderBox* renderer = m_rendererInFlowThread;
-    m_rendererInFlowThread = 0;
-    flowThread()->flowThreadDescendantWillBeRemoved(renderer);
-    // |this| should be destroyed by now.
-}
-
 void RenderMultiColumnSpannerPlaceholder::updateMarginProperties()
 {
     RefPtr<RenderStyle> newStyle = RenderStyle::clone(style());