Introduce AudioDecoderConfig to migrate away from GetAVStream().

media/filters/ffmpeg_demuxer.cc

Index: media/filters/ffmpeg_demuxer.cc
diff --git a/media/filters/ffmpeg_demuxer.cc b/media/filters/ffmpeg_demuxer.cc
index e4e21af700b45f241f606cc8ac90dba58b4e4722..3d7bb12b561a043a8b3210f9ef64e600987e74b1 100644
--- a/media/filters/ffmpeg_demuxer.cc
+++ b/media/filters/ffmpeg_demuxer.cc
@@ -67,6 +67,7 @@ FFmpegDemuxerStream::FFmpegDemuxerStream(FFmpegDemuxer* demuxer,
   switch (stream->codec->codec_type) {
     case AVMEDIA_TYPE_AUDIO:
       type_ = AUDIO;
+      AVCodecContextToAudioDecoderConfig(stream->codec, &audio_config_);
       break;
     case AVMEDIA_TYPE_VIDEO:
       type_ = VIDEO;
@@ -241,6 +242,11 @@ AVStream* FFmpegDemuxerStream::GetAVStream() {
   return stream_;
 }
 
+const AudioDecoderConfig& FFmpegDemuxerStream::audio_decoder_config() {
+  CHECK_EQ(type_, AUDIO);
+  return audio_config_;
+}
+
 // static
 base::TimeDelta FFmpegDemuxerStream::ConvertStreamTimestamp(
     const AVRational& time_base, int64 timestamp) {