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

Source/core/rendering/RenderMultiColumnFlowThread.cpp

Index: Source/core/rendering/RenderMultiColumnFlowThread.cpp
diff --git a/Source/core/rendering/RenderMultiColumnFlowThread.cpp b/Source/core/rendering/RenderMultiColumnFlowThread.cpp
index fcf0f3affb67448b9087849f6841599662ec927d..e44bfdb6a500ff6f43c2c01e83f19428b9aba1ea 100644
--- a/Source/core/rendering/RenderMultiColumnFlowThread.cpp
+++ b/Source/core/rendering/RenderMultiColumnFlowThread.cpp
@@ -83,6 +83,12 @@ void RenderMultiColumnFlowThread::evacuateAndDestroy()
     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();
 }