Backport some tests in /content/browser/renderer_host/media 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
/content/browser/renderer_host/media.

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: Iad0205309ee97970a5f2d2585a2a44a825e872fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786952
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@{#693605}

content/browser/renderer_host/media/audio_input_stream_handle_unittest.cc

@@ -122,7 +122,7 @@ class AudioInputStreamHandleTest : public Test {
   }
 
  private:
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   StrictMock<MockRendererAudioInputStreamFactoryClient> client_;
   mojom::RendererAudioInputStreamFactoryClientPtr client_ptr_;
   mojo::Binding<mojom::RendererAudioInputStreamFactoryClient> client_binding_;