Add jamescook@ to ui/chromeos/OWNERS

BUG=None

Change-Id: I8d0d23a9947b5e4eb56ba02e16c1561076276f08
Reviewed-on: https://chromium-review.googlesource.com/695747Reviewed-by: Dan Erat <derat@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505855}

ui/chromeos/OWNERS

 derat@chromium.org
+jamescook@chromium.org
 oshima@chromium.org
 stevenjb@chromium.org