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

Source/core/rendering/RenderLayer.h

Index: Source/core/rendering/RenderLayer.h
diff --git a/Source/core/rendering/RenderLayer.h b/Source/core/rendering/RenderLayer.h
index 9c8e84886ffed85c0ada141630e03023e528798b..809be71c96874ec3a90e59f696309d318205687a 100644
--- a/Source/core/rendering/RenderLayer.h
+++ b/Source/core/rendering/RenderLayer.h
@@ -676,6 +676,13 @@ private:
     OwnPtr<TransformationMatrix> m_transform;
 
     // Pointer to the enclosing RenderLayer that caused us to be paginated. It is 0 if we are not paginated.
+    //
+    // See RenderMultiColumnFlowThread and
+    // https://sites.google.com/a/chromium.org/dev/developers/design-documents/multi-column-layout
+    // for more information about the multicol implementation. It's important to understand the
+    // difference between flow thread coordinates and visual coordinates when working with multicol
+    // in RenderLayer, since RenderLayer is one of the few places where we have to worry about the
+    // visual ones. Internally we try to use flow-thread coordinates whenever possible.
     RenderLayer* m_enclosingPaginationLayer;
 
     // These compositing reasons are updated whenever style changes, not while updating compositing layers.