Allow any unsignalled SSRC changes on default video receive channel.

The first unsignalled SSRC creates a default receive channel.
Any unsignalled SSRC changes after that replace the default SSRC.
Add unit tests for changing unsignalled SSRCs.

BUG=webrtc:5208

Review-Url: https://codereview.webrtc.org/2692993009
Cr-Commit-Position: refs/heads/master@{#16682}
diff --git a/AUTHORS b/AUTHORS
index 1c23c3e..4ff9c39 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -26,6 +26,7 @@
 Matthias Liebig <matthias.gcode@gmail.com>
 Maxim Potapov <vopatop.skam@gmail.com>
 Mike Gilbert <floppymaster@gmail.com>
+Mo Zanaty <mzanaty@cisco.com>
 Pali Rohar
 Paul Kapustin <pkapustin@gmail.com>
 Philipp Hancke <philipp.hancke@googlemail.com>