This simplifies current_compile_job_ since ScheduleCompileJobForNewUnits
is only called on the main thread.

From pinpoint:
v8:wasm:sync_instantiate:wall_time: 19.1% improvement
v8-gc-incremental-step: 20.5% improvement
https://pinpoint-dot-chromeperf.appspot.com/job/152920d8520000

Change-Id: Id560080937f5439cf3321ce9306c7cae49e74798
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2442383
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70386}