AudioCodingModule: Specify decoders using SdpAudioFormat

NetEq already uses SdpAudioFormat internally; this CL adds an
AudioCodingModule::RegisterReceiveCodec overload that accepts
SdpAudioFormat, and propagates it through AcmReceiver into NetEq.

The intention is to get rid of the other ways to specify decoders and
always use SdpAudioFormat. (And eventually to do the same for encoders
too.)

NOTRY=true
BUG=5801

Review-Url: https://codereview.webrtc.org/2365653004
Cr-Commit-Position: refs/heads/master@{#14506}

webrtc/modules/audio_coding/neteq/decoder_database.h

@@ -145,6 +145,11 @@ class DecoderDatabase {
                               NetEqDecoder codec_type,
                               const std::string& name);
 
+  // Registers a decoder for the given payload type. Returns kOK on success;
+  // otherwise an error code.
+  virtual int RegisterPayload(int rtp_payload_type,
+                              const SdpAudioFormat& audio_format);
+
   // Registers an externally created AudioDecoder object, and associates it
   // as a decoder of type |codec_type| with |rtp_payload_type|.
   virtual int InsertExternal(uint8_t rtp_payload_type,