GitLab

Add two tests to verify the claim in the WallClockTimer documentation.

1) Restart a running timer will replace the current task and reset the
desired run time.
2) Stop a non-running timer is a no-op.

Bug: 1062410
Change-Id: Id5f9b7cc7d55db66cccf085d8260965633a23a7a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2144414
Commit-Queue: Owen Min <zmin@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758899}