This CL refactors ConcurrentMarking::Stop to have explicit Join and Pause.
MarkCompact updates job priority to UserBlocking before joining.

Change-Id: I71cb469e35cc4df7fdb0dbd8c0cf9c1642e8f5fd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2491109Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70738}