arc: Fix chrome/browser/chromeos/arc/process/OWNERS

cmtm@ primarily uses the other account.

BUG=None
TEST=None

Change-Id: I12ec23590fa3a8744d7717b63a7cda94b5ef2c52
Reviewed-on: https://chromium-review.googlesource.com/c/1355922Reviewed-by: Christopher Morin <cmtm@google.com>
Commit-Queue: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612410}

chrome/browser/chromeos/arc/process/OWNERS

-cmtm@chromium.org
+cmtm@google.com