CreateJob() doesn't schedule anything until Join() or Notify*() is called. CreateJob().Join() will thus schedule the right number of
workers for the job right away (taking into account the main thread
contributes), whereas PostJob().Join() schedules 1 worker that won't
be necessary once doing Join() and the main thread kicks in.
This has the effect of reducing 1 unnecessary context switch each time
the jobs are schedule.

Bug: chromium:1287665
Change-Id: Ie262f8904cc8ac78d9e5cbd23ef28dc5b013a625
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3746080Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82047}