[scroll-animations] Use shortest serialization of animation-timeline

Per cssom guidelines, serializations should take the shortest
form possible without changing the meaning. Since "foo" and foo
refer to the same timeline name, and foo is a valid <custom-ident>,
the correct serialization of "foo" is foo.

This matches the behavior of Firefox.

Bug: 1382876
Change-Id: I41c351da1ba75fc8fb63c31de971d0fcb7ee3fad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4061351
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1076199}
diff --git a/scroll-animations/css/animation-timeline-computed.html b/scroll-animations/css/animation-timeline-computed.html
index 68ba1f2..29486a0 100644
--- a/scroll-animations/css/animation-timeline-computed.html
+++ b/scroll-animations/css/animation-timeline-computed.html
@@ -22,16 +22,17 @@
 test_computed_value('animation-timeline', 'none, none');
 test_computed_value('animation-timeline', 'auto, none');
 test_computed_value('animation-timeline', 'none, auto');
-test_computed_value('animation-timeline', '"test"', ["test", '"test"']);
+test_computed_value('animation-timeline', '"test"', 'test');
 test_computed_value('animation-timeline', '"none"');
 test_computed_value('animation-timeline', '"auto"');
 test_computed_value('animation-timeline', '"initial"');
 test_computed_value('animation-timeline', '"inherit"');
 test_computed_value('animation-timeline', '"unset"');
 test_computed_value('animation-timeline', '"revert"');
+test_computed_value('animation-timeline', '"revert-layer"');
 test_computed_value('animation-timeline', 'test');
 test_computed_value('animation-timeline', 'test1, test2');
-test_computed_value('animation-timeline', 'test1, "test2", none, test3, auto', ["test1, test2, none, test3, auto", 'test1, "test2", none, test3, auto']);
+test_computed_value('animation-timeline', 'test1, "test2", none, test3, auto', 'test1, test2, none, test3, auto');
 
 test(() => {
   let style = getComputedStyle(document.getElementById('target'));