Add toLayoutMultiColumnFlowThread().

Source/core/layout/LayoutMultiColumnSet.h

 /*
  * 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 60 matching lines @@
     bool isOfType(LayoutObjectType type) const override { return type == LayoutObjectLayoutMultiColumnSet || LayoutBlockFlow::isOfType(type); }
     bool canHaveChildren() const final { return false; }
 
     // Return the width and height of a single column or page in the set.
     LayoutUnit pageLogicalWidth() const { return flowThread()->logicalWidth(); }
     LayoutUnit pageLogicalHeight() const;
 
     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;
 
     LayoutUnit logicalTopInFlowThread() const;
     LayoutUnit logicalBottomInFlowThread() const;
     LayoutUnit logicalHeightInFlowThread() const { return logicalBottomInFlowThread() - logicalTopInFlowThread(); }
     LayoutRect flowThreadPortionRect() const;
     LayoutRect flowThreadPortionOverflowRect() const;
     LayoutRect overflowRectForFlowThreadPortion(const LayoutRect& flowThreadPortionRect, bool isFirstPortion, bool isLastPortion) const;
@@ skipping 77 matching lines @@
     MultiColumnFragmentainerGroupList m_fragmentainerGroups;
     LayoutFlowThread* m_flowThread;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutMultiColumnSet, isLayoutMultiColumnSet());
 
 } // namespace blink
 
 #endif // LayoutMultiColumnSet_h