Add pending tree lock for worklet animations with a scroll timeline

Preserve the flow of time for scroll-timeline based animations by
preventing active tree mutations while there is a pending tree mutation
awaiting activation.

Bug: 937975
Change-Id: I1bc5ee57fc59a8f8c75f11de36614a8c30a1ac6d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1615627
Reviewed-by: Majid Valipour <majidvp@chromium.org>
Commit-Queue: Kevin Ellis <kevers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663231}
3 files changed