Expose:
- JobHandle::IsCompleted()
- worker_count passed as argument to GetMaxConcurrency()
- JobDelegate::GetTaskId
With default implementation. Once gin implements it, it can be made pure
virtual.

Bug: chromium:1114823
Change-Id: I24ce60d1df6adff4061c050e5aa8bf8d7bb1cf5a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2352485
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69388}