[New Multicolumn] Minimal support for nested multicol in RenderLayer.

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 76 matching lines @@
 // RenderMultiColumnSet::recordSpaceShortage()). We'll relayout (to find new break points and the
 // new lowest amount of space increase that could affect where they occur, in case we need another
 // round) until we've reached an acceptable height (where everything fits perfectly in the number of
 // columns that we have specified). The rule of thumb is that we shouldn't have to perform more of
 // such iterations than the number of columns that we have.
 //
 // For each layout iteration done for column balancing, the flow thread will need a deep layout if
 // column heights changed in the previous pass, since column height changes may affect break points
 // and pagination struts anywhere in the tree, and currently no way exists to do this in a more
 // optimized manner.
+//
+// There's also some documentation online:
+// https://sites.google.com/a/chromium.org/dev/developers/design-documents/multi-column-layout
 class RenderMultiColumnFlowThread : 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()); }
 
@@ skipping 71 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