Help sweeper tasks complete sweeping sooner but do not refill free
lists on shutdown.

This races with allocating background threads. Background threads will
refill free lists themselves if more memory is required.

Bug: v8:10315
Change-Id: Ie615983229701e8c9434b4352bd055e9dbbb8671
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2297466Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68842}