If DefaultJobState::Join is called before any worker tasks were spawned
(e.g. right after Platform::CreateJob), it should spawn the required
number of worker tasks (mimicing what Platform::PostJob followed by Join
would do, but with less context switches).
This fixes regressions we got from switching from Platform::PostJob to
Platform::CreateJob.

R=mlippautz@chromium.org
CC=etiennep@chromium.org

Bug: chromium:1348512
Change-Id: Ic7984d12a28fc67f4b2f51ddc2ba5a406e43c127
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3804600Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82178}