Allow a received audio codec's payload type to change.

webrtc/api/audio_codecs/audio_format.h

Index: webrtc/api/audio_codecs/audio_format.h
diff --git a/webrtc/api/audio_codecs/audio_format.h b/webrtc/api/audio_codecs/audio_format.h
index 8814a3dcdcc0c700424f7990c1c6d8b0261c08ae..4bdb924727a4267c83964045242898315620cdab 100644
--- a/webrtc/api/audio_codecs/audio_format.h
+++ b/webrtc/api/audio_codecs/audio_format.h
@@ -41,6 +41,11 @@ struct SdpAudioFormat {
                  const Parameters& param);
   ~SdpAudioFormat();
 
+  // Returns true if this format is compatible with |o|. In SDP terminology,
+  // would it represent the same codec between an offer and an answer? As
+  // opposed to operator==, this method disregards codec parameters.
+  bool Matches(const SdpAudioFormat& o) const;

[kwiberg-webrtc, 2017/04/21 12:49:00]

Is that a good idea in general? Opus communicates the number of channels in the
parameters, for example.

[Taylor Brandstetter, 2017/04/21 13:02:06]

From an SDP perspective, it does what I intend it to. You can change the number
of channels negotiated through parameters without requiring a new payload type.

[kwiberg-webrtc, 2017/04/21 13:18:57]

Oh---if the RFCs say that that's the requirement, then great! Provided it
actually works, of course. :-) But I guess it will work for Opus at least.

+
   SdpAudioFormat& operator=(const SdpAudioFormat&);
   SdpAudioFormat& operator=(SdpAudioFormat&&);