Add toLayoutMultiColumnFlowThread().

Source/core/layout/LayoutMultiColumnSet.h

Index: Source/core/layout/LayoutMultiColumnSet.h
diff --git a/Source/core/layout/LayoutMultiColumnSet.h b/Source/core/layout/LayoutMultiColumnSet.h
index 0eecd1453b7e025f7eb2b5df004f49f61c491b0b..2f49c33d44a8b3fd1b5a2bab12e5a3006cce9cd4 100644
--- a/Source/core/layout/LayoutMultiColumnSet.h
+++ b/Source/core/layout/LayoutMultiColumnSet.h
@@ -78,11 +78,7 @@ public:
     LayoutFlowThread* flowThread() const { return m_flowThread; }
 
     LayoutBlockFlow* multiColumnBlockFlow() const { return toLayoutBlockFlow(parent()); }
-    LayoutMultiColumnFlowThread* multiColumnFlowThread() const
-    {
-        ASSERT_WITH_SECURITY_IMPLICATION(!flowThread() || flowThread()->isLayoutMultiColumnFlowThread());
-        return static_cast<LayoutMultiColumnFlowThread*>(flowThread());
-    }
+    LayoutMultiColumnFlowThread* multiColumnFlowThread() const { return toLayoutMultiColumnFlowThread(flowThread()); }
 
     LayoutMultiColumnSet* nextSiblingMultiColumnSet() const;
     LayoutMultiColumnSet* previousSiblingMultiColumnSet() const;