[Sheriff] Revert "[Sheriff] skip flaky test on OS 10.12"

This reverts commit fbecbcdf791e8e2f20da9c3fc638d0bfa4891dc4.

Reason for revert: The builder Mac10.12 has been failing for
over a month. There is no need to skip this test on 10.12.
Once the builder is fixed the issue will probably be gone.

Original change's description:
> [Sheriff] skip flaky test on OS 10.12
>
> Bug: 1299873
> Change-Id: Iad971c5702a11ec59c8b5a6e4972bbd463fc2825
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3481879
> Owners-Override: Dana Fried <dfried@google.com>
> Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
> Commit-Queue: Dana Fried <dfried@chromium.org>
> Auto-Submit: Dana Fried <dfried@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#973951}

Bug: 1299873
Change-Id: I5d516b11935ae4e1ea9b9974528f4e47eb2348de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3489953
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Owners-Override: Renato Silva <rrsilva@google.com>
Auto-Submit: Renato Silva <rrsilva@google.com>
Reviewed-by: Fabian Sommer <fabiansommer@chromium.org>
Commit-Queue: Fabian Sommer <fabiansommer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#975084}