Add ',' between elements in RTCStatsReport::ToJson

There was no ',' between array-elements the stats-report, yielding
invalid JSON. Let's bring back the ",", which was lost during recent
refactoring.

Bug: webrtc:10173
Change-Id: Ib58025d56c4895c6af33b9777cb2ebdb94a678ea
Reviewed-on: https://webrtc-review.googlesource.com/c/116483
Reviewed-by: Oleh Prypin <oprypin@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Oleh Prypin <oprypin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26161}
diff --git a/AUTHORS b/AUTHORS
index 1216b48..dc79326 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -17,6 +17,7 @@
 David Porter <david@porter.me>
 Dax Booysen <dax@younow.com>
 Danail Kirov <dkirovbroadsoft@gmail.com>
+Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
 Dmitry Lizin <sdkdimon@gmail.com>
 Eric Rescorla, RTFM Inc. <ekr@rtfm.com>
 Frederik Riedel, Frogg GmbH <frederik.riedel@frogg.io>