GitLab

Grew tired of the post+wait pattern in many tests to flush a base::Thread.
Was going to need it again in https://codereview.chromium.org/2333023003/
and decided to add an API for everyone's sake instead :-).

Off the top of my head, will also be useful in
https://codereview.chromium.org/2299523003/

BUG=646443

Review-Url: https://codereview.chromium.org/2337253003
Cr-Commit-Position: refs/heads/master@{#419026}