Fix cast_media_unittests

Since 0467279e ("Reland "[Chromecast] Fix thread safety issues in
CastAudioJson."") we need to use a full task environment to avoid a CHECK
failure calling base::CreateSequencedTaskRunner().

Bug: 143135800
Test: cast_media_unittests
Change-Id: Ib07489414e4b13c74a7441a0f6d3fb4d73f6351c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1874808
Auto-Submit: Michael Spang <spang@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708635}

chromecast/media/cma/backend/multizone_backend_unittest.cc

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