Backport some tests in /chromeos/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/audio.

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

This CL was uploaded by git cl split.

R=jennyz@chromium.org

Bug: 891670
Change-Id: I3cd6813e7cfab55d4c4d0e1d9d249677154ca9c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787062
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@{#694565}

chromeos/audio/cras_audio_handler_unittest.cc

@@ -308,7 +308,8 @@ class FakeVideoCaptureManager {
 class CrasAudioHandlerTest : public testing::TestWithParam<int> {
  public:
   CrasAudioHandlerTest()
-      : task_environment_(base::test::TaskEnvironment::MainThreadType::UI) {}
+      : task_environment_(
+            base::test::SingleThreadTaskEnvironment::MainThreadType::UI) {}
   ~CrasAudioHandlerTest() override = default;
 
   void SetUp() override {
@@ -483,7 +484,7 @@ class CrasAudioHandlerTest : public testing::TestWithParam<int> {
     return FakeCrasAudioClient::Get();
   }
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   base::SystemMonitor system_monitor_;
   SystemMonitorObserver system_monitor_observer_;
   CrasAudioHandler* cras_audio_handler_ = nullptr;         // Not owned.