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..093a1a1f1250d96586350563edd3555a2c010721 100644
--- a/media/filters/dummy_demuxer.cc
+++ b/media/filters/dummy_demuxer.cc
@@ -18,6 +18,10 @@ DemuxerStream::Type DummyDemuxerStream::type() {
   return type_;
 }
 
+AudioDecoderConfig* DummyDemuxerStream::audio_decoder_config() {
+  return NULL;
+}
+
 void DummyDemuxerStream::Read(const ReadCallback& read_callback) {}
 
 void DummyDemuxerStream::EnableBitstreamConverter() {}