GitLab

1) Reorder the function in .cpp to match the header file.
2) Add a unit test for Stop() function.
3) When time is up, remove the observer before running the |user_task|.
   This allows user restart the timer inside the |user_task|.

Bug: 1062410
Change-Id: I6472c97198d63f4745a6ee5f2af7f376c8555894
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2140715Reviewed-by: Greg Thompson <grt@chromium.org>
Commit-Queue: Owen Min <zmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757829}