StopRequest is removed in favor of:
COMPLETE_TASKS_FOR_TESTING -> JoinForTesting()
PREEMPT_TASKS -> Pause()
COMPLETE_ONGOING_TASKS now has the same behavior as PREEMPT_TASKS
- we should avoid waiting on the main thread as much as possible.

Change-Id: Icceeb4f0c0fda2ed234b2f26fe308b11410fcfb7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2376166
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70037}