Re-add AC3/EAC3 audio demuxing support

media/ffmpeg/ffmpeg_common.cc

Index: media/ffmpeg/ffmpeg_common.cc
diff --git a/media/ffmpeg/ffmpeg_common.cc b/media/ffmpeg/ffmpeg_common.cc
index c9d866968050ec3d660ce7ef07e7abce75182434..20aa2200c20c56b2de8c4e9e6aaa99655b81beaa 100644
--- a/media/ffmpeg/ffmpeg_common.cc
+++ b/media/ffmpeg/ffmpeg_common.cc
@@ -67,6 +67,12 @@ static AudioCodec CodecIDToAudioCodec(AVCodecID codec_id) {
   switch (codec_id) {
     case AV_CODEC_ID_AAC:
       return kCodecAAC;
+#if defined(ENABLE_AC3_EAC3_AUDIO_DEMUXING)
+    case AV_CODEC_ID_AC3:
+      return kCodecAC3;
+    case AV_CODEC_ID_EAC3:
+      return kCodecEAC3;
+#endif
     case AV_CODEC_ID_MP3:
       return kCodecMP3;
     case AV_CODEC_ID_VORBIS:
@@ -293,8 +299,11 @@ bool AVCodecContextToAudioDecoderConfig(const AVCodecContext* codec_context,
 
   AudioCodec codec = CodecIDToAudioCodec(codec_context->codec_id);
 
+  // For AC3/EAC3, FFmpeg does not fill |sample_fmt|.
   SampleFormat sample_format =
-      AVSampleFormatToSampleFormat(codec_context->sample_fmt);
+      (codec == kCodecAC3 || codec == kCodecEAC3)
+          ? kSampleFormatS16
+          : AVSampleFormatToSampleFormat(codec_context->sample_fmt);
 
   ChannelLayout channel_layout = ChannelLayoutToChromeChannelLayout(
       codec_context->channel_layout, codec_context->channels);
@@ -343,8 +352,7 @@ bool AVCodecContextToAudioDecoderConfig(const AVCodecContext* codec_context,
                      is_encrypted,
                      seek_preroll,
                      codec_context->delay);
-
-  if (codec != kCodecOpus) {
+  if (codec != kCodecOpus && codec != kCodecAC3 && codec != kCodecEAC3) {
     DCHECK_EQ(av_get_bytes_per_sample(codec_context->sample_fmt) * 8,
               config->bits_per_channel());
   }