Remove references of MessageLoopForIO in /chromecast/media/cma/backend/fuchsia

MessageLoopForIO will go away soon use ScopedTaskEnvironment instead.

ScopedTaskEnvironment will per default start a ThreadPool, which should
be fine in most of the cases. If you believe your test needs to make sure
that no ThreadPool runs let me know and I will update the patch.

BUG=891670
This CL was uploaded by git cl split.

R=jamesr@chromium.org

Change-Id: I4ab402a4a2d79a4e65461ad7ebb00d67a4011899
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1733492
Commit-Queue: Carlos Caballero <carlscab@google.com>
Reviewed-by: Wez <wez@chromium.org>
Auto-Submit: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#684036}

chromecast/media/cma/backend/fuchsia/BUILD.gn

@@ -55,6 +55,7 @@ cast_source_set("unit_tests") {
   deps = [
     ":cma_backend_support",
     "//base",
+    "//base/test:test_support",
     "//testing/gtest",
     "//third_party/fuchsia-sdk/sdk:media",
   ]

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

@@ -5,8 +5,8 @@
 #include "chromecast/media/cma/backend/fuchsia/mixer_output_stream_fuchsia.h"
 
 #include "base/location.h"
-#include "base/message_loop/message_loop.h"
 #include "base/single_thread_task_runner.h"
+#include "base/test/scoped_task_environment.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ -18,7 +18,9 @@ constexpr int kNumChannels = 2;
 
 class MixerOutputStreamFuchsiaTest : public ::testing::Test {
  protected:
-  base::MessageLoopForIO message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_{
+      base::test::ScopedTaskEnvironment::ThreadingMode::MAIN_THREAD_ONLY,
+      base::test::ScopedTaskEnvironment::MainThreadType::IO};
   MixerOutputStreamFuchsia output_;
 };