Backport some tests in /media/base/audio_renderer_mixer_input_unittest.cc to...

Backport some tests in /media/base/audio_renderer_mixer_input_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/audio_renderer_mixer_input_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: Ib96c66f6f7c392fec6344c517439234b2d8bc045
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786945
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@{#693604}

media/base/audio_renderer_mixer_input_unittest.cc

@@ -109,7 +109,7 @@ class AudioRendererMixerInputTest : public testing::Test,
  protected:
   ~AudioRendererMixerInputTest() override = default;
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   AudioParameters audio_parameters_;
   std::unique_ptr<AudioRendererMixer> mixers_[2];
   scoped_refptr<AudioRendererMixerInput> mixer_input_;