[scheduling] Use ScopedTaskEnvironment instead of MessageLoop in chrome_cleaner

MessageLoop will go away, eventually.

Also fixes a race condition in ChromeCleanerRunnerTest that was perturbed by
the scheduler change.

BUG=891670

Change-Id: Ice4336cec91f337e8aea0145e25eb99d8602806a
Reviewed-on: https://chromium-review.googlesource.com/c/1464112
Reviewed-by: Chris Sharp <csharp@chromium.org>
Commit-Queue: Joe Mason <joenotcharles@google.com>
Cr-Commit-Position: refs/heads/master@{#631228}
2 files changed