The signature of {GetMaxConcurrency} was recently extended to pass the
{worker_count}, i.e. the number of workers that are currently running.
This number allows us to return a more precise number for the current
maximum concurrency.
In the case of background function compilation, we were sometimes
returning a slightly too small number, resulting in too few workers to
be spawned, resulting in slightly longer compilation.
For wrapper compilation on the other hand, the returned number is
already correct, and this CL adds a comment explaining why.

R=thibaudm@chromium.org

Bug: chromium:1101340
Change-Id: I0e3122c8b99ba1cdf97616de922d4f07874b0aeb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2410383Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69906}