Support async mutations in cc scheduler.

* Defer tick of animation worklets until draw to allow time for pending mutations to complete.
* Add plumbing for notifying scheduler that mutations are pending.
* Block activation and immediate draw if there are pending mutations.

See also https://chromium-review.googlesource.com/c/chromium/src/+/1409600
Remaining step: Plumb pending/ready state from mutator dispatcher through to cc scheduler.


Bug: 791280
Change-Id: I3d3f33b5b069853771207b902ff97baf347364a3
Reviewed-on: https://chromium-review.googlesource.com/c/1412513
Commit-Queue: Kevin Ellis <kevers@chromium.org>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629234}
10 files changed