[FilesApp] Add Austin to OWNERS

Austin continues to make valuable contributions to FilesApp. He has a
very good grasp of the FilesApp code base, good testing focus, and is
asked to review more changes given his knowledge of FilesApp. Time to
make him an OWNER.

No-try: true
Bug: None
Change-Id: Id7a55dda4034de8e4a4b6c1f2cf60e4648ffff5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1726801Reviewed-by: Anand Mistry <amistry@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682578}

ui/file_manager/OWNERS

 amistry@chromium.org
+austinct@chromium.org
 dats@chromium.org
 fdegros@chromium.org
 fukino@chromium.org