fix issue 1322, accept -1 as default payload-type for redundant coding (FEC).
fix issue 1322, accept -1 as default payload-type for redundant coding (FEC).

issue=1322 
test=trybot, voe auto-tes
Review URL: https://webrtc-codereview.appspot.com/1043007

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 8c8ad85c5d70c38cea82f5b17794a4e7ab4cf531

sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc