Use ScopedTaskEnvironment instead of MessageLoop in /chromeos/process_proxy

MessageLoop will go away, eventually.

ScopedTaskEnvironment will per default start a ThreadPool, which should
be fine in most of the cases. If you belive 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=tbarzic@chromium.org

Change-Id: Ib2a2cd41a6a01184a2876bcc099f4c49879dcb39
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1648206
Auto-Submit: Carlos Caballero <carlscab@google.com>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Commit-Queue: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#669596}
1 file changed