This CL is adding a WorkerPool delegate to allow an embedder
to post task on a custom thread pool. The target for this code
is Chromium.

The plan is to post tasks into the Chromium ThreadPool.

Related CLs:
1) [this]
2) https://chromium-review.googlesource.com/c/chromium/src/+/2231864
3) https://chromium-review.googlesource.com/c/angle/angle/+/2231710

Bug: chromium:1091259
Change-Id: Ib990b06d4672b6f859d04b97ac4311a7a80ef7a9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2231708
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>