Remove {available_task_ids_} and use the task ID provided by the
JobDelegate instead.

R=clemensb@chromium.org

Bug: chromium:1123471
Change-Id: I62c7efd68fbb0a93ac3d36614b3248b60da65ff8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2400986
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69788}