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}
2 files changed