No longer any need to delete the line box tree on our own.

Source/core/layout/LayoutMultiColumnFlowThread.cpp

 /*
  * 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 237 matching lines @@
 
     ASSERT(!previousSibling());
     ASSERT(!nextSibling());
 
     // Finally we can promote all flow thread's children. Before we move them to the flow thread's
     // container, we need to unregister the flow thread, so that they aren't just re-added again to
     // the flow thread that we're trying to empty.
     multicolContainer->resetMultiColumnFlowThread();
     moveAllChildrenTo(multicolContainer, true);
 
-    // FIXME: it's scary that neither destroy() nor the move*Children* methods take care of this,
-    // and instead leave you with dangling root line box pointers. But since this is how it is done
-    // in other parts of the code that deal with reparenting layoutObjects, let's do the cleanup on our
-    // own here as well.
-    deleteLineBoxTree();
+    // We used to manually nuke the line box tree here, but that should happen automatically when
+    // moving children around (the code above).
+    ASSERT(!firstLineBox());
 
     destroy();
 }
 
 LayoutSize LayoutMultiColumnFlowThread::columnOffset(const LayoutPoint& point) const
 {
     if (!hasValidColumnSetInfo())
         return LayoutSize(0, 0);
 
     LayoutPoint flowThreadPoint = flipForWritingMode(point);
@@ skipping 692 matching lines @@
     if (LayoutMultiColumnSet* multicolSet = columnSetAtBlockOffset(offset)) {
         multicolSet->addContentRun(offset);
         if (offsetBreakAdjustment)
             *offsetBreakAdjustment = pageLogicalHeightForOffset(offset) ? pageRemainingLogicalHeightForOffset(offset, AssociateWithFormerPage) : LayoutUnit();
         return true;
     }
     return false;
 }
 
 }