Remove references of MessageLoopForIO in /media/filters/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: I6c82de2bdf0d0c819c5c185788f727b0be5dbe87
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1733590
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Auto-Submit: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#684124}

media/filters/fuchsia/fuchsia_video_decoder_unittest.cc

@@ -6,7 +6,7 @@
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
-#include "base/message_loop/message_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "media/base/test_data_util.h"
 #include "media/base/test_helpers.h"
 #include "media/base/video_decoder.h"
@@ -73,7 +73,9 @@ class FuchsiaVideoDecoderTest : 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};
   std::unique_ptr<VideoDecoder> decoder_;
 
   std::list<scoped_refptr<VideoFrame>> output_frames_;