Backport some tests in /chromeos/process_proxy to SingleThreadTaskEnvironment

These tests were previously migrated from single-threaded MessageLoop to
a multi-threaded TaskEnvironment (then named ScopedTaskEnvironment) as
part of crbug.com/891670.

//base OWNERS decided in retrospect that it was better to keep a
single-threaded option for TaskEnvironment and introduced
SingleThreadTaskEnvironment. This CL retrofits that decision for
/chromeos/process_proxy.

This CL is a no-op if it passes CQ.

This CL was uploaded by git cl split.

R=tbarzic@chromium.org

Bug: 891670
Change-Id: I925c2fd509220c100983f1745568056029251af1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786687
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694552}

• chromeos/process_proxy/process_output_watcher_unittest.cc[diff]