[New Multicolumn] Add RenderMultiColumnFlowThread::findSetRendering().

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 106 matching lines @@
     }
     // Return the last column set or spanner placeholder.
     RenderBox* lastMultiColumnBox() const
     {
         RenderBox* lastSiblingBox = multiColumnBlockFlow()->lastChildBox();
         // The flow thread is the first child of the multicol container. If the flow thread is also
         // the last child, it means that there are no siblings; i.e. we have no column boxes.
         return lastSiblingBox != this ? lastSiblingBox : 0;
     }
 
+    // Find the first set inside which the specified renderer would be rendered.
+    RenderMultiColumnSet* findSetRendering(RenderObject*) const;
+
     // Return the spanner placeholder that belongs to the spanner in the containing block chain, if
     // any. This includes the renderer for the element that actually establishes the spanner too.
     RenderMultiColumnSpannerPlaceholder* containingColumnSpannerPlaceholder(const RenderObject* descendant) const;
 
     // Populate the flow thread with what's currently its siblings. Called when a regular block
     // becomes a multicol container.
     void populate();
 
     // Empty the flow thread by moving everything to the parent. Remove all multicol specific
     // renderers. Then destroy the flow thread. Called when a multicol container becomes a regular
@@ skipping 52 matching lines @@
     LayoutUnit m_columnHeightAvailable; // Total height available to columns, or 0 if auto.
     bool m_inBalancingPass; // Set when relayouting for column balancing.
     bool m_needsColumnHeightsRecalculation; // Set when we need to recalculate the column set heights after layout.
     bool m_progressionIsInline; // Always true for regular multicol. False for paged-y overflow.
     bool m_isBeingEvacuated;
 };
 
 } // namespace blink
 
 #endif // RenderMultiColumnFlowThread_h