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

Source/core/rendering/RenderMultiColumnFlowThread.h

Index: Source/core/rendering/RenderMultiColumnFlowThread.h
diff --git a/Source/core/rendering/RenderMultiColumnFlowThread.h b/Source/core/rendering/RenderMultiColumnFlowThread.h
index 778325cd72897ef4782696a28c23443465a22701..24fe07169e6992e67d6fc6e0723221f75733a1c1 100644
--- a/Source/core/rendering/RenderMultiColumnFlowThread.h
+++ b/Source/core/rendering/RenderMultiColumnFlowThread.h
@@ -94,6 +94,9 @@ class RenderMultiColumnSpannerPlaceholder;
 // 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();