[web-animations-2] [scroll-animations] setting an animation's timeline to a progress-based timeline should create a new `ready` promise

[graouts]

Step 9 of the setting the timeline of an animation procedure deals with the case where the new timeline is progress-based is to set the auto align start time flag to true. When this flag is true, the animation's start time may be computed to a new value based on the associated timeline's current time.

Furthermore, this procedures schedules a pending play task if the animation had finished or was running.

Since the start time will be set when the timeline is updated next, it seems to me that we should also reset the ready promise in the case where we schedule a pending play task.

In fact, some existing scroll-animations tests actually expect this behavior:

• scroll-animations/css/animation-timeline-ignored.tentative.html
• scroll-animations/scroll-timelines/setting-timeline.tentative.html

Chrome passes those tests and seems to implement this behavior. I think this is an oversight in the spec.

[graouts]

Cc @kevers-google, @flackr, @andruud and @bramus.