While waiting for baseline compilation to finish (i.e. in the
synchronous compilation API), do also contribute to compilation. This
restores the performance from before switching to the jobs API, because
all threads will now be used for synchronous compilation.

Drive-by: Remove unused {CompilationStateImpl::background_compile_token}
method.

R=ahaas@chromium.org

Bug: chromium:1101340, chromium:1113234
Change-Id: I61eddb813eb2779cdd41f11c8d840d63132d4360
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2351671Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69373}