Backport some tests in /services/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
/services/audio.

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

This CL was uploaded by git cl split.

R=miu@chromium.org

Bug: 891670
Change-Id: Ie208338463113dfc3e0542765156760b0aaaf6ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786855
Auto-Submit: Gabriel Charette <gab@chromium.org>
Commit-Queue: Olga Sharonova <olka@chromium.org>
Reviewed-by: Olga Sharonova <olka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695139}

services/audio/test/audio_system_to_service_adapter_test.cc

@@ -69,7 +69,7 @@ class AudioSystemToServiceAdapterTestBase : public testing::Test {
   // AudioSystem conformance tests won't set expecnations.
   NiceMock<MockFunction<void(void)>> system_info_bind_requested_;
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   std::unique_ptr<media::MockAudioManager> audio_manager_;
   std::unique_ptr<mojom::SystemInfo> system_info_impl_;
   std::unique_ptr<mojo::Receiver<mojom::SystemInfo>> system_info_receiver_;
@@ -433,8 +433,8 @@ class AudioSystemToServiceAdapterDisconnectTest : public testing::Test {
   MOCK_METHOD0(ClientConnected, void(void));
   MOCK_METHOD0(ClientDisconnected, void(void));
 
-  base::test::TaskEnvironment task_environment_{
-      base::test::TaskEnvironment::TimeSource::MOCK_TIME};
+  base::test::SingleThreadTaskEnvironment task_environment_{
+      base::test::SingleThreadTaskEnvironment::TimeSource::MOCK_TIME};
 
   const base::Optional<std::string> valid_reply_{kValidReplyId};
   base::MockCallback<media::AudioSystem::OnDeviceIdCallback> response_received_;