Introduce AudioDecoderConfig to migrate away from GetAVStream().

media/ffmpeg/ffmpeg_common.h

Index: media/ffmpeg/ffmpeg_common.h
diff --git a/media/ffmpeg/ffmpeg_common.h b/media/ffmpeg/ffmpeg_common.h
index dec47dfaa7910ddeaa73dfd979652dc8cbbb55b0..d917b92d839707a73acc348ec1bdcce6a4ca4053 100644
--- a/media/ffmpeg/ffmpeg_common.h
+++ b/media/ffmpeg/ffmpeg_common.h
@@ -11,6 +11,7 @@
 #include "base/compiler_specific.h"
 #include "base/memory/singleton.h"
 #include "base/time.h"
+#include "media/base/audio_decoder_config.h"
 #include "media/base/channel_layout.h"
 #include "media/base/media_export.h"
 #include "media/video/video_decode_engine.h"
@@ -65,6 +66,13 @@ MEDIA_EXPORT base::TimeDelta ConvertFromTimeBase(const AVRational& time_base,
 MEDIA_EXPORT int64 ConvertToTimeBase(const AVRational& time_base,
                                      const base::TimeDelta& timestamp);
 
+void AVCodecContextToAudioDecoderConfig(
+    const AVCodecContext* codec_context,
+    AudioDecoderConfig* config);
+void AudioDecoderConfigToAVCodecContext(
+    const AudioDecoderConfig& config,
+    AVCodecContext* codec_context);
+
 VideoCodec CodecIDToVideoCodec(CodecID codec_id);
 CodecID VideoCodecToCodecID(VideoCodec video_codec);