Introduce AudioDecoderConfig to migrate away from GetAVStream().

media/filters/dummy_demuxer.cc

Index: media/filters/dummy_demuxer.cc
diff --git a/media/filters/dummy_demuxer.cc b/media/filters/dummy_demuxer.cc
index 91aa40d188f475015c2d47fddfcf0eae603f41e7..a9afbfcbf38b9b4a3af1bb6319c67f92a60e6e83 100644
--- a/media/filters/dummy_demuxer.cc
+++ b/media/filters/dummy_demuxer.cc
@@ -18,6 +18,11 @@ DemuxerStream::Type DummyDemuxerStream::type() {
   return type_;
 }
 
+const AudioDecoderConfig& DummyDemuxerStream::audio_decoder_config() {
+  CHECK_EQ(type_, AUDIO);
+  return audio_config_;
+}
+
 void DummyDemuxerStream::Read(const ReadCallback& read_callback) {}
 
 void DummyDemuxerStream::EnableBitstreamConverter() {}