Backport some tests in /chromecast/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
/chromecast/media.

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

This CL was uploaded by git cl split.

R=kmackay@chromium.org

Bug: 891670
Change-Id: I762f13951f43e2675273a3e364c06dee2977d072
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786852
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694572}

chromecast/media/base/media_resource_tracker_unittest.cc

@@ -72,7 +72,7 @@ class MediaResourceTrackerTest : public ::testing::Test {
     base::RunLoop().RunUntilIdle();
   }
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   TestMediaResourceTracker* resource_tracker_;
   std::unique_ptr<MediaResourceTrackerTestMocks> test_mocks_;
 

chromecast/media/cma/backend/multizone_backend_unittest.cc

@@ -152,7 +152,7 @@ class MultizoneBackendTest : public testing::TestWithParam<TestParams> {
   void OnEndOfStream();
 
  private:
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   std::vector<std::unique_ptr<BufferFeeder>> effects_feeders_;
   std::unique_ptr<BufferFeeder> audio_feeder_;