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}