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..37f0e98832c272ea91602a3f124edf6484e036b8 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;
+      audio_config_.reset(AVCodecContextToAudioDecoderConfig(stream->codec));
       break;
     case AVMEDIA_TYPE_VIDEO:
       type_ = VIDEO;
@@ -241,6 +242,10 @@ AVStream* FFmpegDemuxerStream::GetAVStream() {
   return stream_;
 }
 
+AudioDecoderConfig* FFmpegDemuxerStream::audio_decoder_config() {
+  return audio_config_.get();
+}
+
 // static
 base::TimeDelta FFmpegDemuxerStream::ConvertStreamTimestamp(
     const AVRational& time_base, int64 timestamp) {