Use ScopedTaskEnvironment instead of MessageLoop

MessageLoop will go away, eventually.

ScopedTaskEnvironment will per default start a ThreadPool, which should
be fine in most of the cases. If you belive 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=chcunningham@chromium.org

Change-Id: I78fc74ba3ed1e3441c130d8426c557b18a470fb4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1635619
Commit-Queue: Chrome Cunningham <chcunningham@chromium.org>
Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Auto-Submit: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#665277}

• media/base/android/media_player_bridge_unittest.cc[diff]
• media/base/android/media_service_throttler_unittest.cc[diff]
• media/base/bind_to_current_loop_unittest.cc[diff]
• media/base/fake_demuxer_stream_unittest.cc[diff]
• media/base/null_video_sink_unittest.cc[diff]
• media/base/pipeline_impl_unittest.cc[diff]
• media/base/serial_runner_unittest.cc[diff]
• media/base/text_renderer_unittest.cc[diff]
• media/device_monitors/system_message_window_win_unittest.cc[diff]
• media/filters/aom_video_decoder_unittest.cc[diff]
• media/filters/audio_decoder_unittest.cc[diff]
• media/filters/dav1d_video_decoder_unittest.cc[diff]
• media/filters/decrypting_audio_decoder_unittest.cc[diff]
• media/filters/decrypting_demuxer_stream_unittest.cc[diff]
• media/filters/decrypting_video_decoder_unittest.cc[diff]
• media/filters/fake_video_decoder_unittest.cc[diff]
• media/filters/ffmpeg_video_decoder_unittest.cc[diff]
• media/filters/frame_processor_unittest.cc[diff]
• media/filters/pipeline_controller_unittest.cc[diff]
• media/filters/video_decoder_stream_unittest.cc[diff]
• media/mojo/clients/mojo_android_overlay_unittest.cc[diff]
• media/mojo/clients/mojo_audio_decoder_unittest.cc[diff]
• media/mojo/clients/mojo_video_encode_accelerator_unittest.cc[diff]
• media/mojo/common/mojo_data_pipe_read_write_unittest.cc[diff]
• media/mojo/common/mojo_decoder_buffer_converter_unittest.cc[diff]
• media/mojo/services/mojo_video_encode_accelerator_service_unittest.cc[diff]
• media/mojo/services/watch_time_recorder_unittest.cc[diff]
• media/mojo/test/mojo_video_encode_accelerator_integration_test.cc[diff]
• media/renderers/paint_canvas_video_renderer_unittest.cc[diff]
• media/renderers/renderer_impl_unittest.cc[diff]
• media/renderers/video_resource_updater_unittest.cc[diff]