Use ScopedTaskEnvironment instead of MessageLoopForUI in /chrome/chrome_cleaner

MessageLoopForUI will go away, eventually.

ScopedTaskEnvironment will per default start a ThreadPool, which should
be fine in most of the cases. If you believe your test needs to make sure
that no ThreadPool runs let me know and I will update the patch.

BUG=891670
This CL was uploaded by git cl split.

R=csharp@chromium.org

Change-Id: I61d4664fd9c12eeb17083bb6ede19ce6d1d22087
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715299
Auto-Submit: Carlos Caballero <carlscab@google.com>
Reviewed-by: Joe Mason <joenotcharles@google.com>
Commit-Queue: Joe Mason <joenotcharles@google.com>
Cr-Commit-Position: refs/heads/master@{#680921}