[fieldtrial] Add Mac platform to V8ConcurrentSparkplug

Bug: v8:12054
Change-Id: I0dff54cf8b293fc844bd0e40d3f40346cf3b9697
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3890930


Auto-Submit: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1045775}