arc: Annotate arc/OWNERS files

BUG=None
TEST=No code change

Change-Id: I1627f25269b4eeac33d78674a515aeeab028a124
Reviewed-on: https://chromium-review.googlesource.com/c/1343069Reviewed-by: Elijah Taylor <elijahtaylor@chromium.org>
Commit-Queue: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609553}

chrome/browser/chromeos/arc/OWNERS

-elijahtaylor@chromium.org
 hidehiko@chromium.org
 yusukes@chromium.org
+# backup reviewer
+elijahtaylor@chromium.org

components/arc/OWNERS

-elijahtaylor@chromium.org
 hidehiko@chromium.org
 yusukes@chromium.org
+# backup reviewer
+elijahtaylor@chromium.org