Stopped using the NetEqDecoder enum internally in NetEq.

webrtc/modules/audio_coding/neteq/include/neteq.h

Index: webrtc/modules/audio_coding/neteq/include/neteq.h
diff --git a/webrtc/modules/audio_coding/neteq/include/neteq.h b/webrtc/modules/audio_coding/neteq/include/neteq.h
index c07143c062020dd9fd02d46843cf550fd82168e4..8520905b1097b1e6e0c13b7ce57dcfd8ee58275d 100644
--- a/webrtc/modules/audio_coding/neteq/include/neteq.h
+++ b/webrtc/modules/audio_coding/neteq/include/neteq.h
@@ -258,10 +258,10 @@ class NetEq {
   // value if we have no decoder for that payload type.
   virtual rtc::Optional<CodecInst> GetDecoder(int payload_type) const = 0;
 
-  // Returns the decoder format for the given payload type. Returns null if no
-  // such payload type was registered, or if it was registered without
-  // providing an SdpAudioFormat.
-  virtual const SdpAudioFormat* GetDecoderFormat(int payload_type) const = 0;
+  // Returns the decoder format for the given payload type. Returns empty if no
+  // such payload type was registered.
+  virtual rtc::Optional<SdpAudioFormat> GetDecoderFormat(
+      int payload_type) const = 0;
 
   // Not implemented.
   virtual int SetTargetNumberOfChannels() = 0;