layout: Let stretch on flex item cross size stretch to the line (#38526)

We were instead stretching to the containing block, which implied that
the behaviors of a `stretch` size and `stretch` alignment weren't
consistent.

As resolved by the CSSWG, the behavior will now be:
 - If the cross size of the line is known, stretch to the line.
 - Otherwise, stretch to the containing block.

See https://github.com/w3c/csswg-drafts/issues/11784

This aligns us with Blink, which has already shipped this new behavior.

Testing: Improves existing WPT and adds a new test.

Signed-off-by: Oriol Brufau <obrufau@igalia.com>

components/layout/geom.rs

@@ -993,7 +993,7 @@ impl From<Option<Au>> for SizeConstraint {
     }
 }
 
-#[derive(Clone, Default)]
+#[derive(Clone, Debug, Default)]
 pub(crate) struct Sizes {
     /// <https://drafts.csswg.org/css-sizing-3/#preferred-size-properties>
     pub preferred: Size<Au>,