Add backer@ to ui/gl/OWNERS

Change-Id: I06a35cb28512af457eb7e77fe44eedfef7fab9f9
Reviewed-on: https://chromium-review.googlesource.com/c/1359055Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Jonathan Backer <backer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613195}

ui/gl/OWNERS

+backer@chromium.org
 kbr@chromium.org
 piman@chromium.org
 per-file *gl_image*=reveman@chromium.org