Make all one-line flexboxes size their line to the flexbox, per WG discussion.

--HG--
extra : rebase_source : 59fcb36bb89109f6bbaf4a284267f0e2fa4278fc

Author: tabatkins

css3-flexbox/Overview.src.html

@@ -1646,7 +1646,16 @@ <h3 id='cross-sizing'>
 	<ol start=8>
 		<li id='algo-cross-line'>
 			<strong>Calculate the cross size of each flexbox line.</strong>
-			For each flexbox line:
+
+			<p>
+				If the flexbox has only a single line
+				(even if it's a <i>multi-line</i> flexbox),
+				the cross size of the flexbox line
+				is the flexbox's inner cross size.
+
+			<p>
+				Otherwise,
+				for each flexbox line:
 
 			<ol>
 				<li>