arc: Updates OWNERS for arc session code.

Updates to existing OWNERS file for c/arc/session, and adds a new
OWNERS file for c/b/c/a/session.

BUG=None
TEST=n/a

Change-Id: Id81e8a537afc86d8a1aeeaaec7cba3c82a8e1f9b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2124537Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Commit-Queue: Josh Horwich <jhorwich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754517}

chrome/browser/chromeos/arc/session/OWNERS

+file://components/arc/session/OWNERS

components/arc/session/OWNERS

+ereth@chromium.org
+lgcheng@google.com
+# backup reviewers
 hashimoto@chromium.org
 hidehiko@chromium.org
+jhorwich@chromium.org
 yusukes@chromium.org