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 f817547006fbfdd5ffe887d7a4abd573a6ca9dd5..370a6dceb0c2adb369f817623aadb243d86ebcff 100644
--- a/media/ffmpeg/ffmpeg_common.cc
+++ b/media/ffmpeg/ffmpeg_common.cc
@@ -65,6 +65,12 @@ 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:
@@ -279,8 +285,11 @@ void AVCodecContextToAudioDecoderConfig(
 
   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);
@@ -316,7 +325,7 @@ void AVCodecContextToAudioDecoderConfig(
                      record_stats,
                      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());
   }