Since Sparkplug compiles pretty quickly and it might impact loading
time, there is an argument that we should actually use high priority
threads for CSP.

This adds a flag so that we create a finch experiment to test this
hypothesis.

Change-Id: Ib8965fbea015ddaeb25503bd92873bfff5daa1ce
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3605245
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Auto-Submit: Victor Gomes <victorgomes@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80251}