Backport some tests in /chromecast/media/cma/backend/fuchsia 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
/chromecast/media/cma/backend/fuchsia.

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

This CL was uploaded by git cl split.

R=wez@chromium.org

Bug: 891670
Change-Id: I64b5b78848e49d1219e77f4006ca5f770dbe38ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786799
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694557}

chromecast/media/cma/backend/fuchsia/mixer_output_stream_fuchsia_unittest.cc

@@ -18,9 +18,8 @@ constexpr int kNumChannels = 2;
 
 class MixerOutputStreamFuchsiaTest : public ::testing::Test {
  protected:
-  base::test::TaskEnvironment task_environment_{
-      base::test::TaskEnvironment::ThreadingMode::MAIN_THREAD_ONLY,
-      base::test::TaskEnvironment::MainThreadType::IO};
+  base::test::SingleThreadTaskEnvironment task_environment_{
+      base::test::SingleThreadTaskEnvironment::MainThreadType::IO};
   MixerOutputStreamFuchsia output_;
 };