Changing between multicol and regular block shouldn't recreate all renderers.

Source/core/rendering/RenderMultiColumnFlowThread.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 20 matching lines @@
 
 namespace WebCore {
 
 class RenderMultiColumnFlowThread FINAL : public RenderFlowThread {
 public:
     virtual ~RenderMultiColumnFlowThread();
 
     static RenderMultiColumnFlowThread* createAnonymous(Document&, RenderStyle* parentStyle);
 
     RenderBlockFlow* multiColumnBlockFlow() const { return toRenderBlockFlow(parent()); }
+
+    // Populate the flow thread with what's currently its siblings. Called when a regular block
+    // becomes a multicol container.
+    void populate();
+
+    // Empty the flow thread by moving everything to the parent. Remove all multicol specific
+    // renderers. Then destroy the flow thread. Called when a multicol container becomes a regular
+    // block.
+    void evacuateAndDestroy();
+
     unsigned columnCount() const { return m_columnCount; }
     LayoutUnit columnWidth() const { return m_columnWidth; }
     LayoutUnit columnHeightAvailable() const { return m_columnHeightAvailable; }
     void setColumnHeightAvailable(LayoutUnit available) { m_columnHeightAvailable = available; }
     bool requiresBalancing() const { return !columnHeightAvailable() || multiColumnBlockFlow()->style()->columnFill() == ColumnFillBalance; }
 
     void layoutColumns(bool relayoutChildren, SubtreeLayoutScope&);
     bool computeColumnCountAndWidth();
     bool recalculateColumnHeights();
 
@@ skipping 13 matching lines @@
     LayoutUnit m_columnWidth; // The used value of column-width
     LayoutUnit m_columnHeightAvailable; // Total height available to columns, or 0 if auto.
     bool m_inBalancingPass; // Set when relayouting for column balancing.
     bool m_needsRebalancing;
 };
 
 } // namespace WebCore
 
 #endif // RenderMultiColumnFlowThread_h