[css-grid-2] Add scrollbar gutter to mbp list w3c#9935

Author: fantasai

css-grid-2/Overview.bs

@@ -3853,7 +3853,7 @@ Subgrids</h2>
 
 		<li id="subgrid-edge-placeholders">
 			For each edge of a non-empty [=subgrid=],
-			to account for the subgrid’s margin/border/padding at that edge,
+			to account for the subgrid’s margin/border/padding (and any scrollbar gutter) at that edge,
 			a hypothetical item is contributed to the track sizing algorithm
 			for each span size
 			in the set of items spanning into