The refactoring is triggered by https://crrev.com/c/3121905 where we
noticed that a bunch of tricky counter paths could be simplified,
making reasoning about corectness easier.

In this CL:
1. Use uniqe_ptr instead of Optional to allow moving SweepingJob away
   from the header file.
2. sweeping_in_progress_ is replaced with simply checking for a job.
3. freed_bytes_ are moved to the job and the dependency is reversed,
   avoiding the inside-out (Job->Sweeper) dependency completely.
4. Merge() and counter updates are merged into a Finalize() method.
5. FinishIfDone() allows for conditional finization.
6. young_bytes_ and old_bytes_ are removed as they were always updated
   when the corresponding bytes in the ArrayBufferList was updated.

Change-Id: I56e5b04087166ce03d3a9195ac48359122a84c73
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3124776Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76795}