Backport some tests in /dbus 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
/dbus.

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

This CL was uploaded by git cl split.

R=hashimoto@chromium.org

Bug: 891670
Change-Id: I05ed8a9c81b692d2deda1ebb32abfce3011f15cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786754
Auto-Submit: Gabriel Charette <gab@chromium.org>
Commit-Queue: Ryo Hashimoto <hashimoto@chromium.org>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#694664}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 1858a23ce09a71969577c9c29b73231d7c795d90
6 files changed