[css-animations-2] Make animation-duration:auto resolve to 0s for legacy compat #6530

[fantasai]

See #6530

[birtles]

Looks good to me but I should probably defer to someone more familiar with animation-timeline to review. (Also, I assume someone is making WPT for this case.)

[andruud]

I assume someone is making WPT

Yes, I'll do that.

[flackr]

Looks good to me. This resolves the most important blocking case that when neither animation-timeline nor animation-duration are set we get 0s in the computed style.