Cancel animation when a custom Timeline is used (#35567)

Follow up to #35559.

The clean up function of the custom timeline doesn't necessarily clean
up the animation. Just the timeline's internal state.

This affects Firefox which doesn't support ScrollTimeline so uses the
polyfill's custom timeline.

fixtures/view-transition/src/components/Page.js

@@ -150,6 +150,8 @@ export default function Page({url, navigate}) {
       const cleanup1 = timeline.animate(animation1);
       const cleanup2 = timeline.animate(animation2);
       return () => {
+        animation1.cancel();
+        animation2.cancel();
         cleanup1();
         cleanup2();
       };