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Currently when scroll timeline has unresolved current time, e.g. the
scroll source is not scrollable, we set WorkletAnimation::start_time_ to
base::TimeDelta::FromSecondsD(NaN) which is incorrect.

Other changes in the patch:
1. By spec [1], unresolved time values are represented by null. Update
tests to match that.
2. For WorkletAnimation with ScrollTimeline, it's legit not to set
start_time_. Update DCHECKs for this behavior.

[1] https://drafts.csswg.org/web-animations/#time-values-in-the-programming-interface

Bug: 905358
Change-Id: Ifde3890a6a4ad9e27dda8d4a78c5b6dcbf24ef54
Reviewed-on: https://chromium-review.googlesource.com/c/1336055
Commit-Queue: Yi Gu <yigu@chromium.org>
Reviewed-by: Majid Valipour <majidvp@chromium.org>
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608841}