[animation-worklet] Enable wpt tests to run with threaded compostiting


Bug: 915352
Change-Id: Iead91d55465c0277dfaa9588d413e654a5d1a57a
Reviewed-on: https://chromium-review.googlesource.com/c/1418076
Commit-Queue: Majid Valipour <majidvp@chromium.org>
Reviewed-by: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623836}