Optimize player sorting.
Optimize player sorting.

* Sort by sequence number per spec.
* Don't require a stable sort.
* Track and invalidate the sorted state.

birtles/web-animations-js,beni55/web-animations-js-legacy,web-animations/web-animations-js-legacy,web-animations/web-animations-js-legacy,beni55/web-animations-js-legacy,web-animations/web-animations-js-legacy,beni55/web-animations-js-legacy,birtles/web-animations-js,beni55/web-animations-js-legacy,birtles/web-animations-js