Backport some tests in /chromeos/dbus/audio 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/dbus/audio.

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

This CL was uploaded by git cl split.

R=hychao@chromium.org

Bug: 891670
Change-Id: I55a04b25e80b979196259ec1c593cad76481163f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786960
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Jenny Zhang <jennyz@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695331}

chromeos/dbus/audio/cras_audio_client_unittest.cc

@@ -437,7 +437,7 @@ class CrasAudioClientTest : public testing::Test {
   // The interface name.
   const std::string interface_name_;
   // A message loop to emulate asynchronous behavior.
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   // The mock bus.
   scoped_refptr<dbus::MockBus> mock_bus_;
   // The mock object proxy.