Implemented setPlaybackRate for scroll-linked Worklet Animations.

• Updated logic of calculating initial current time of non-composited scroll-linked animations to
  ensure that the start time is always initialized to zero.
• Changed signature of cc::ScrollTimeline::CurrentTime to return base::Optional<base::TimeTicks>.
  This is to enforce cc: WorkletAnimation::CurrentTime operating in the same units of
  base::TimeTicks and correctly handling NaN values.
• Adjusted start time of scroll-linked animations when playback rate is updated.


Bug: 852475
Change-Id: Iffd966c1b3ef6d821eef7cc2f6f79dae790a69f3
Reviewed-on: https://chromium-review.googlesource.com/c/1461513
Commit-Queue: Olga Gerchikov <[email protected]>
Reviewed-by: Stephen McGruer <[email protected]>
Reviewed-by: Majid Valipour <[email protected]>
Cr-Commit-Position: refs/heads/master@{#636836}
10 files changed