Backport some tests in /media/base/fake_audio_worker_unittest.cc 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
/media/base/fake_audio_worker_unittest.cc.

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: I0e22fd1759c690657cebc7f7677c6942cd27a461
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786742
Auto-Submit: Gabriel Charette <gab@chromium.org>
Commit-Queue: Yuri Wiitala <miu@chromium.org>
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695803}

media/base/fake_audio_worker_unittest.cc

@@ -87,8 +87,8 @@ class FakeAudioWorkerTest : public testing::Test {
   }
 
  protected:
-  base::test::TaskEnvironment task_environment_{
-      base::test::TaskEnvironment::TimeSource::MOCK_TIME};
+  base::test::SingleThreadTaskEnvironment task_environment_{
+      base::test::SingleThreadTaskEnvironment::TimeSource::MOCK_TIME};
   AudioParameters params_;
   FakeAudioWorker fake_worker_;
   base::TimeDelta time_between_callbacks_;