[css-contain] Update test coverage info

Author: frivoal

css-contain-1/Overview.bs

@@ -160,6 +160,7 @@ Strong Containment: the 'contain' property</h2>
 				contain-size-010.html
 				contain-size-011.html
 				contain-size-012.html
+				contain-size-012b.html
 				contain-size-013.html
 				contain-size-021.html
 				contain-size-023.html
@@ -423,6 +424,7 @@ Size Containment</h3>
 	contain-size-009.html
 	contain-size-010.html
 	contain-size-012.html
+	contain-size-012b.html
 	contain-size-051.html
 	contain-size-052.html
 	</wpt>