We sometimes create jobs that initially hold no work. In those cases,
use CreateJob instead of PostJob.
New background threads will later be spawned when
NotifyConcurrencyIncrease is called.

R=etiennep@chromium.org

Bug: v8:13096
Change-Id: Ieb9f9e03d01af6a72fe5785be72c523a553d0f1d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3762578Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82060}