Update WebCodecs OWNERS

This CL adds two WebCodecs contributors to owners.

Change-Id: I6e6f00dc40bb4b52eb941a656a428cbca1f1440c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2357916
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Auto-Submit: Thomas Guilbert <tguilbert@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798927}

third_party/blink/renderer/modules/webcodecs/OWNERS

@@ -2,3 +2,4 @@ mlamouri@chromium.org
 sandersd@chromium.org
 dalecurtis@chromium.org
 chcunningham@chromium.org
+tguilbert@chromium.org
\ No newline at end of file