Backport some tests in...

Backport some tests in /media/mojo/services/mojo_audio_output_stream_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/mojo/services/mojo_audio_output_stream_unittest.cc.

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

This CL was uploaded by git cl split.

R=tommi@chromium.org

Bug: 891670
Change-Id: I8b0eab2befd64fcaa60ee99ae87db72ec3999839
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787238
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Tommi <tommi@chromium.org>
Commit-Queue: Tommi <tommi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693602}

media/mojo/services/mojo_audio_output_stream_unittest.cc

@@ -171,7 +171,7 @@ class MojoAudioOutputStreamTest : public Test {
         .WillOnce(SaveArg<0>(&delegate_event_handler_));
   }
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   base::CancelableSyncSocket local_;
   std::unique_ptr<TestCancelableSyncSocket> foreign_socket_;
   base::UnsafeSharedMemoryRegion mem_;