This reverts commit f4b41099.

Reason for revert: Several GC failures, e.g. https://build.chromium.org/p/client.v8/builders/V8%20Linux/builds/23236, https://build.chromium.org/p/client.v8/builders/V8%20Mac/builds/18390 

Original change's description:
> Introduce ConcurrentMarking::StopRequest API.
> 
> 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}

TBR=gab@chromium.org,ulan@chromium.org,mlippautz@chromium.org

Change-Id: Ia001cc81c6a7bc030b54d3aa9b9bcecc833300e6
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:812178
Reviewed-on: https://chromium-review.googlesource.com/925302Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51365}