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

Source/core/rendering/RenderBlockFlow.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2007 David Smith (catfish.man@gmail.com)
  * Copyright (C) 2003-2013 Apple Inc. All rights reserved.
  * Copyright (C) Research In Motion Limited 2010. All rights reserved.
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
@@ skipping 179 matching lines @@
     static TextRun constructTextRun(RenderObject* context, const Font&, const RenderText*, unsigned offset, RenderStyle*,
         TextRun::ExpansionBehavior = TextRun::AllowTrailingExpansion | TextRun::ForbidLeadingExpansion);
 
     static TextRun constructTextRun(RenderObject* context, const Font&, const LChar* characters, int length, RenderStyle*, TextDirection,
         TextRun::ExpansionBehavior = TextRun::AllowTrailingExpansion | TextRun::ForbidLeadingExpansion);
 
     static TextRun constructTextRun(RenderObject* context, const Font&, const UChar* characters, int length, RenderStyle*, TextDirection,
         TextRun::ExpansionBehavior = TextRun::AllowTrailingExpansion | TextRun::ForbidLeadingExpansion);
 
     RenderMultiColumnFlowThread* multiColumnFlowThread() const { return m_rareData ? m_rareData->m_multiColumnFlowThread : 0; }
+    void resetMultiColumnFlowThread()
+    {
+        if (m_rareData)
+            m_rareData->m_multiColumnFlowThread = 0;
+    }
 
     void addOverflowFromInlineChildren();
 
     // FIXME: This should be const to avoid a const_cast, but can modify child dirty bits and RenderCombineText
     void computeInlinePreferredLogicalWidths(LayoutUnit& minLogicalWidth, LayoutUnit& maxLogicalWidth);
 
     GapRects inlineSelectionGaps(RenderBlock* rootBlock, const LayoutPoint& rootBlockPhysicalPosition, const LayoutSize& offsetFromRootBlock,
         LayoutUnit& lastLogicalTop, LayoutUnit& lastLogicalLeft, LayoutUnit& lastLogicalRight, const PaintInfo*);
 protected:
     void rebuildFloatsFromIntruding();
@@ skipping 84 matching lines @@
     LayoutUnit logicalRightOffsetForPositioningFloat(LayoutUnit logicalTop, LayoutUnit fixedOffset, bool applyTextIndent, LayoutUnit* heightRemaining) const;
     LayoutUnit logicalLeftOffsetForPositioningFloat(LayoutUnit logicalTop, LayoutUnit fixedOffset, bool applyTextIndent, LayoutUnit* heightRemaining) const;
 
     LayoutUnit adjustLogicalRightOffsetForLine(LayoutUnit offsetFromFloats, bool applyTextIndent) const;
     LayoutUnit adjustLogicalLeftOffsetForLine(LayoutUnit offsetFromFloats, bool applyTextIndent) const;
 
     virtual void adjustForBorderFit(LayoutUnit x, LayoutUnit& left, LayoutUnit& right) const OVERRIDE; // Helper function for borderFitAdjust
 
     virtual RootInlineBox* createRootInlineBox(); // Subclassed by SVG
 
-    void createMultiColumnFlowThreadIfNeeded();
+    void createOrDestroyMultiColumnFlowThreadIfNeeded();
 
     void updateLogicalWidthForAlignment(const ETextAlign&, const RootInlineBox*, BidiRun* trailingSpaceRun, float& logicalLeft, float& totalLogicalWidth, float& availableLogicalWidth, int expansionOpportunityCount);
     void checkForPaginationLogicalHeightChange(LayoutUnit& pageLogicalHeight, bool& pageLogicalHeightChanged, bool& hasSpecifiedPageLogicalHeight);
     bool shouldRelayoutForPagination(LayoutUnit& pageLogicalHeight, LayoutUnit layoutOverflowLogicalBottom) const;
     void setColumnCountAndHeight(unsigned count, LayoutUnit pageLogicalHeight);
 
 public:
     struct FloatWithRect {
         FloatWithRect(RenderBox* f)
             : object(f)
@@ skipping 174 matching lines @@
 
 // END METHODS DEFINED IN RenderBlockLineLayout
 
 };
 
 DEFINE_RENDER_OBJECT_TYPE_CASTS(RenderBlockFlow, isRenderBlockFlow());
 
 } // namespace WebCore
 
 #endif // RenderBlockFlow_h