CreateJob() is similar to PostJob() but doesn't schedule anything
until Join() or Notify*() is called.
This allows
- CreateJob().Join() without too many worker.
- Early 'empty' CreateJob() for initialization
  without causing spurious calls to GetMaxConcurrency()

Bug: chromium:1287665
Change-Id: I8fd8b139392ad30218f0cf8f580b2d76f1078777
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3668842Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81166}