[animation-worklet] Make timeline & options optional in WorkletAnimation constructor

The last two values should be optional according to the specification [1].
Added two new factory methods to allow this. To do this cleanly
switched WorkletAnimation to have Member<AnimationTimeline> instead of
holding onto a DocumentTimelineOrScrollTimeline.

[1] https://wicg.github.io/animation-worklet/#creating-worklet-animation

TEST: virtual/threaded/fast/animationworklet/worklet-animation-creation.html
Bug: 840383
Change-Id: Ida5aa077fbf19b4918652c02e92f40435db4fbb4
Reviewed-on: https://chromium-review.googlesource.com/1047691
Commit-Queue: Majid Valipour <majidvp@chromium.org>
Reviewed-by: Majid Valipour <majidvp@chromium.org>
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559240}
8 files changed