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

Source/core/rendering/RenderFlowThread.h

Index: Source/core/rendering/RenderFlowThread.h
diff --git a/Source/core/rendering/RenderFlowThread.h b/Source/core/rendering/RenderFlowThread.h
index c9668079d658748fab9363fbeda6163552970339..285089bd807d8d9fe5f02729e1254e8bcb1f0b0b 100644
--- a/Source/core/rendering/RenderFlowThread.h
+++ b/Source/core/rendering/RenderFlowThread.h
@@ -59,6 +59,11 @@ public:
 
     virtual bool isRenderFlowThread() const OVERRIDE FINAL { return true; }
 
+#ifndef NDEBUG
+    // This method is only used in assertions at the moment.
+    virtual bool isRenderMultiColumnFlowThread() const { return false; }
+#endif // NDEBUG
+
     virtual void layout() OVERRIDE FINAL;
 
     // Always create a RenderLayer for the RenderFlowThread so that we