This was extracted from https://chromium-review.googlesource.com/c/v8/v8/+/924073/10
after it became clear that using COMPLETE_TASKS/PREEMPT_TASKS where
it should make sense to doesn't work in practice for now.

Experimental CLs which led to the above conclusion:
 - https://chromium-review.googlesource.com/c/v8/v8/+/924865
   (COMPLETE or CANCEL -- still broken)
 - https://chromium-review.googlesource.com/c/v8/v8/+/924866
   (CANCEL only, as before, works)
 - https://chromium-review.googlesource.com/c/v8/v8/+/924028
   (CANCEL and PREEMPT -- broken as well)

Introducing this unittested API allows to reduce the size
of the CLs causing hard-to-diagnose bots-only failures
and fix them individually follow-ups @

 1) https://chromium-review.googlesource.com/c/v8/v8/+/924029
 2) https://chromium-review.googlesource.com/c/v8/v8/+/924031
 3) https://chromium-review.googlesource.com/c/v8/v8/+/924030

Bug: chromium:812178
Change-Id: Icdac456e9f7874b0c4b321ccdb8898297dad7d73
Reviewed-on: https://chromium-review.googlesource.com/924867
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51353}