So long and thanks for all the code reviews!

- Remove myself from OWNERS.
- Add myself to AUTHORS (I signed a CLA).
- Add minyue to audio_conference_mixer which would otherwise be empty.
- Add missing comma in WATCHLISTS.

Review URL: https://codereview.webrtc.org/1458763002

Cr-Commit-Position: refs/heads/master@{#10686}
diff --git a/AUTHORS b/AUTHORS
index 3ecc088..538b755 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -1,6 +1,7 @@
 # Names should be added to this file like so:
 # Name or Organization <email address>
 
+Andrew MacDonald <andrew@webrtc.org>
 Anil Kumar <an1kumar@gmail.com>
 Ben Strong <bstrong@gmail.com>
 Bob Withers <bwit@pobox.com>