Reland "[wasm] Use NumOutstandingCompilations() in BackgroundCompileJob:GetMaxConcurrency()"

This is a reland of 84eec6e9

Original change's description:
> [wasm] Use NumOutstandingCompilations() in BackgroundCompileJob:GetMaxConcurrency()
>
> 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}

Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel_ng
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_isolates_rel_ng
Change-Id: Ic989b64f130a00ce52228cdd2f57f4c1ade354f2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2458147
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70418}