Use similar strategy to scavenger when calculating number of parallel
compaction tasks. Do not add task for each page but every fourth page.

Bug: chromium:1051883
Change-Id: Iaba3046de9c9a3ab63c0e7afcc6e4a9c398c5a10
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2054097Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66260}