Add tkchin to video_coding OWNERS.

BUG=
R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1217853006.

Cr-Commit-Position: refs/heads/master@{#9528}
diff --git a/webrtc/modules/video_coding/codecs/OWNERS b/webrtc/modules/video_coding/codecs/OWNERS
index 07c2987..37a6e6e 100644
--- a/webrtc/modules/video_coding/codecs/OWNERS
+++ b/webrtc/modules/video_coding/codecs/OWNERS
@@ -1,2 +1,3 @@
 stefan@webrtc.org
 marpan@webrtc.org
+tkchin@webrtc.org