Introduce AudioDecoderConfig to migrate away from GetAVStream().

media/base/demuxer_stream.h

Index: media/base/demuxer_stream.h
diff --git a/media/base/demuxer_stream.h b/media/base/demuxer_stream.h
index 525611561ba68d4209a9665416eba7c7c83b9912..e81423e8069bd3f4abaf902138c167641177d4c2 100644
--- a/media/base/demuxer_stream.h
+++ b/media/base/demuxer_stream.h
@@ -13,6 +13,7 @@ struct AVStream;
 
 namespace media {
 
+class AudioDecoderConfig;
 class Buffer;
 
 class MEDIA_EXPORT DemuxerStream
@@ -34,6 +35,10 @@ class MEDIA_EXPORT DemuxerStream
   // Returns an |AVStream*| if supported, or NULL.
   virtual AVStream* GetAVStream();
 
+  // Returns the audio decoder configuration. It is an error to call this method
+  // if type() != AUDIO.
+  virtual const AudioDecoderConfig& audio_decoder_config() = 0;
+
   // Returns the type of stream.
   virtual Type type() = 0;