[web-animations-2] The handle of unresolved current time in at-progress-timeline-boundary

[BorisChiou]

Based on the step 3 of at-progress-timeline-boundary in [web-animations-2],

Let effective timeline time be animation’s current time / animation’s playback rate + effective start time

we calculate the effective timeline time by animation's current time, playback rate, and the effective start time. However, this spec doesn't mention what should we do for unresolved current time. So perhaps It'd be better to specify what happens when the animation's current time is unresolved.

In Chromium, it uses zero if it is unresolved (code), so for now, Gecko would like to implement this by setting zero for unresolved current time as well.