[New Multicolumn] Destroy line box tree when evacuating the flowthread for multicol.

Source/core/rendering/RenderMultiColumnFlowThread.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 65 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 renderers, let's do the cleanup on our
+    // own here as well.
+    deleteLineBoxTree();
+
     destroy();
 }
 
 void RenderMultiColumnFlowThread::layoutColumns(bool relayoutChildren, SubtreeLayoutScope& layoutScope)
 {
     // Update the dimensions of our regions before we lay out the flow thread.
     // FIXME: Eventually this is going to get way more complicated, and we will be destroying regions
     // instead of trying to keep them around.
     RenderBlockFlow* container = multiColumnBlockFlow();
     bool shouldInvalidateRegions = false;
@@ skipping 171 matching lines @@
 {
     for (RenderBox* renderer = parentBox()->lastChildBox(); renderer; renderer = renderer->previousSiblingBox()) {
         if (renderer->isRenderMultiColumnSet())
             return toRenderMultiColumnSet(renderer)->computedColumnHeight();
     }
     // A column set hasn't been created yet. Height may already be known if column-fill is 'auto', though.
     return !requiresBalancing();
 }
 
 }