The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c023bb9b0a6a898c1296111b10929da95a56f0a2

commit c023bb9b0a6a898c1296111b10929da95a56f0a2
Author: Yi Gu <yigu@chromium.org>
Date: Fri Nov 16 17:42:25 2018

[animation worklet] Do not set start_time_ if scroll timeline has unresolved current time

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}
[modify] https://crrev.com/c023bb9b0a6a898c1296111b10929da95a56f0a2/cc/animation/worklet_animation.cc
[modify] https://crrev.com/c023bb9b0a6a898c1296111b10929da95a56f0a2/third_party/WebKit/LayoutTests/fast/animation/scroll-animations/scrolltimeline-currenttime-nan.html
[modify] https://crrev.com/c023bb9b0a6a898c1296111b10929da95a56f0a2/third_party/WebKit/LayoutTests/fast/animation/scroll-animations/scrolltimeline-currenttime.html
[modify] https://crrev.com/c023bb9b0a6a898c1296111b10929da95a56f0a2/third_party/blink/renderer/core/animation/scroll_timeline.cc
[modify] https://crrev.com/c023bb9b0a6a898c1296111b10929da95a56f0a2/third_party/blink/renderer/core/animation/scroll_timeline_test.cc
[modify] https://crrev.com/c023bb9b0a6a898c1296111b10929da95a56f0a2/third_party/blink/renderer/modules/animationworklet/worklet_animation.cc