Add RTX codecs for codecs only supported by external encoder.

Previously we were only adding these RTX codecs if the codec was
internally supported.

R=pbos@webrtc.org, pthatcher@webrtc.org

Review URL: https://codereview.webrtc.org/2088233004 .

Cr-Original-Commit-Position: refs/heads/master@{#13328}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 6c3e788dcfe694e382fee4d662ca3ea6c1d8c463
2 files changed