Add tguilbert@ to media/OWNERS files!

He's been a committer for a while now and has made many significant
changes around our media subsystems.

Review-Url: https://codereview.chromium.org/2686683003
Cr-Commit-Position: refs/heads/master@{#448788}

media/OWNERS

@@ -15,6 +15,7 @@ hubbe@chromium.org
 jrummell@chromium.org
 liberato@chromium.org
 sandersd@chromium.org
+tguilbert@chromium.org
 watk@chromium.org
 wolenetz@chromium.org
 xhwang@chromium.org