Allow off-thread finalization for parallel compile tasks (i.e. for top-
level IIFEs).

This allows us to merge the code paths in BackgroundCompileTask, and
re-enable the compiler dispatcher tests under the off-thread
finalization flag. Indeed, we can simplify further and get rid of that
flag entirely (it has been on-by-default for several releases now).

Change-Id: I54f361997d651667fa813ec09790a6aab4d26774
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3226780Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77615}