Add toLayoutMultiColumnFlowThread().

Source/core/layout/LayoutMultiColumnFlowThread.h

Index: Source/core/layout/LayoutMultiColumnFlowThread.h
diff --git a/Source/core/layout/LayoutMultiColumnFlowThread.h b/Source/core/layout/LayoutMultiColumnFlowThread.h
index 25e05de7e1d187ebbbf7413c7b9933a4b60cea14..ca36f2ab1b26a8c3c53af5845e570db2da614c75 100644
--- a/Source/core/layout/LayoutMultiColumnFlowThread.h
+++ b/Source/core/layout/LayoutMultiColumnFlowThread.h
@@ -224,6 +224,10 @@ private:
     bool m_isBeingEvacuated;
 };
 
+// Cannot use DEFINE_LAYOUT_OBJECT_TYPE_CASTS here, because isMultiColumnFlowThread() is defined in
+// LayoutFlowThread, not in LayoutObject.
+DEFINE_TYPE_CASTS(LayoutMultiColumnFlowThread, LayoutFlowThread, object, object->isLayoutMultiColumnFlowThread(), object.isLayoutMultiColumnFlowThread());
+
 } // namespace blink
 
 #endif // LayoutMultiColumnFlowThread_h