[New Multicolumn] Faster and more direct RenderMultiColumnSet::multiColumnFlowThread().

Source/core/rendering/RenderMultiColumnFlowThread.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 38 matching lines @@
 // is unbreakable and cannot fully fit in the same column as the preceding piece of
 // content. Although a RenderMultiColumnFlowThread is laid out, it does not take up any space in its
 // container. It's the RenderMultiColumnSet objects that take up the necessary amount of space, and
 // make sure that the columns are painted and hit-tested correctly.
 class RenderMultiColumnFlowThread FINAL : public RenderFlowThread {
 public:
     virtual ~RenderMultiColumnFlowThread();
 
     static RenderMultiColumnFlowThread* createAnonymous(Document&, RenderStyle* parentStyle);
 
+    virtual bool isRenderMultiColumnFlowThread() const OVERRIDE FINAL { return true; }
+
     RenderBlockFlow* multiColumnBlockFlow() const { return toRenderBlockFlow(parent()); }
 
     RenderMultiColumnSet* firstMultiColumnSet() const;
     RenderMultiColumnSet* lastMultiColumnSet() const;
 
     virtual void addChild(RenderObject* newChild, RenderObject* beforeChild = 0) OVERRIDE;
 
     // Populate the flow thread with what's currently its siblings. Called when a regular block
     // becomes a multicol container.
     void populate();
@@ skipping 31 matching lines @@
     LayoutUnit m_columnWidth; // The used value of column-width
     LayoutUnit m_columnHeightAvailable; // Total height available to columns, or 0 if auto.
     bool m_inBalancingPass; // Set when relayouting for column balancing.
     bool m_needsRebalancing;
 };
 
 } // namespace WebCore
 
 #endif // RenderMultiColumnFlowThread_h