Add zmo to ui/gl OWNERS

Bug: None
Change-Id: Icb435aae59f43ace9c52af6a67ddcb331181ad91
Reviewed-on: https://chromium-review.googlesource.com/c/1359318Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613273}

ui/gl/OWNERS

 backer@chromium.org
 kbr@chromium.org
 piman@chromium.org
+zmo@chromium.org
 per-file *gl_image*=reveman@chromium.org
 per-file *gl_image*=dcastagna@chromium.org
 per-file *_ozone*=alexst@chromium.org