[css-grid] Handle items who depend on the height of their track. Related to w3c#1039.

Author: tabatkins

css-grid-1/Overview.bs

@@ -3691,6 +3691,23 @@ Resolve Intrinsic Track Sizes</h3>
 -->
 
 	<ol>
+		<li>
+			<ins>
+
+			<strong>Shim baseline-aligned items
+			so their intrinsic size contributions reflect their baseline alignment.</strong>
+			For the items in each <a>baseline-sharing group</a>,
+			add a “shim” (effectively, additional margin)
+			on the start/end side (for first/last-baseline alignment)
+			of each item
+			so that,
+			when start/end-aligned together
+			their [[css-align-3#baseline-values|baselines align as specified]].
+
+			Consider these “shims” as part of the items’ intrinsic size contribution
+			for the purpose of track sizing, below.
+			</ins>
+
 		<li>
 			<strong>Size tracks to fit non-spanning items:</strong>
 			For each track with an intrinsic <a>track sizing function</a>,