[New Multicolumn] A renderer can't be both a parent and a sibling of another renderer.

Source/core/rendering/RenderBlockFlow.cpp

Index: Source/core/rendering/RenderBlockFlow.cpp
diff --git a/Source/core/rendering/RenderBlockFlow.cpp b/Source/core/rendering/RenderBlockFlow.cpp
index 84aff4e1773ccbdeee660f2284e8a6a517dd8001..b6450a70f7de902c1db987e2c4b4bf276cf5a001 100644
--- a/Source/core/rendering/RenderBlockFlow.cpp
+++ b/Source/core/rendering/RenderBlockFlow.cpp
@@ -1978,6 +1978,9 @@ void RenderBlockFlow::setStaticInlinePositionForChild(RenderBox* child, LayoutUn
 void RenderBlockFlow::addChild(RenderObject* newChild, RenderObject* beforeChild)
 {
     if (RenderMultiColumnFlowThread* flowThread = multiColumnFlowThread()) {
+        if (beforeChild == flowThread)
+            beforeChild = flowThread->firstChild();
+        ASSERT(!beforeChild || beforeChild->isDescendantOf(flowThread));
         flowThread->addChild(newChild, beforeChild);
         return;
     }