Unify parse post-processing between main-thread and background-thread
parsing, now that we have LocalIsolate and can Internalize on background
threads.

As part of this, simplify the LocalIsolate parking pattern to explicitly
park during ParseOnBackground, rather than being implicitly parked when
ParseOnBackground is called. This reduces the amound of scoping needed
in the BackgroundCompileTask::Run method.

Change-Id: Ifdb128b763129bda78bd1bae89dac1c62f872350
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3277876
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77872}