[css-grid] Per f2f, misordered lines now just get swapped rather than dropped, in placement.

Author: tabatkins

css-grid/Overview.bs

@@ -2460,8 +2460,8 @@ Line-based Placement: the 'grid-row-start', 'grid-column-start', 'grid-row-end',
 Grid Placement Conflict Handling</h4>
 
 	If the <a>placement</a> for a <a>grid item</a> contains two lines,
-	and the <a>end</a> line's index is less than or equal to the <a>start</a> line's index,
-	remove the <a>end</a> line from the <a>placement</a>.
+	and the <a>start</a> line is further end-ward than the <a>end</a> line,
+	swap the two lines.
 
 	If the <a>placement</a> contains two spans,
 	remove the one contributed by the <a>end</a> <a>grid-placement property</a>.