Remove DemuxerStream::GetAVStream() once and for all.

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 a9afbfcbf38b9b4a3af1bb6319c67f92a60e6e83..24307b9ef14562bdef95f6b3a353a9c9c8c2784d 100644
--- a/media/filters/dummy_demuxer.cc
+++ b/media/filters/dummy_demuxer.cc
@@ -23,6 +23,11 @@ const AudioDecoderConfig& DummyDemuxerStream::audio_decoder_config() {
   return audio_config_;
 }
 
+const VideoDecoderConfig& DummyDemuxerStream::video_decoder_config() {
+  CHECK_EQ(type_, VIDEO);
+  return video_config_;
+}
+
 void DummyDemuxerStream::Read(const ReadCallback& read_callback) {}
 
 void DummyDemuxerStream::EnableBitstreamConverter() {}