Introduce AudioDecoderConfig to migrate away from GetAVStream().

media/filters/adaptive_demuxer.cc

Index: media/filters/adaptive_demuxer.cc
diff --git a/media/filters/adaptive_demuxer.cc b/media/filters/adaptive_demuxer.cc
index 84a84c8a37bdee40f8b50cf2f2d022cf67a137b0..de980f98cdc09518baba0848d0b853c89e837604 100644
--- a/media/filters/adaptive_demuxer.cc
+++ b/media/filters/adaptive_demuxer.cc
@@ -217,6 +217,10 @@ AVStream* AdaptiveDemuxerStream::GetAVStream() {
   return current_stream()->GetAVStream();
 }
 
+const AudioDecoderConfig& AdaptiveDemuxerStream::audio_decoder_config() {
+  return current_stream()->audio_decoder_config();
+}
+
 DemuxerStream::Type AdaptiveDemuxerStream::type() {
   return current_stream()->type();
 }