This is a reland of 76195d9e.

It was reverted because the new parallel tasks (with higher number
of workers) hang on client.v8.ports bots. Since each test task steals
the worker thread it's assigned but only processes one item before
waiting for completion by others: I think the problem is that there
aren't enough workers in client.v8.ports' config. There aren't any
try bots for this config... reduce the tests to use 4 tasks and
hope for the best (i.e. a 4 core machine that uses "num cores")...

Original change's description:
> Smoother distribution of worker assignment in parallel task array.
>
> This is a merge of https://chromium-review.googlesource.com/c/v8/v8/+/888704
> and https://chromium-review.googlesource.com/c/v8/v8/+/887084
>
> Which implements the fix in CL 887084 correctly in a world where
> there can be more tasks_ than items_ (crbug.com/806237).
>
> Bug: chromium:805932
> Change-Id: I05401be4fdce442644a8973281a9d88bd959b271
> Reviewed-on: https://chromium-review.googlesource.com/892883
> Commit-Queue: Gabriel Charette <gab@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#50956}

Reverted-on: https://chromium-review.googlesource.com/893462

Bug: chromium:805932
Change-Id: I4d0bda3b9f52e9160e613a8f34a95e48b814bb9e
Reviewed-on: https://chromium-review.googlesource.com/893362Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50967}