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 3d3a8b89957d36be73c1480b1a63788dee3049d9..1794df152a3c40a8f0cbbc9d0d352dd45f864c8a 100644
--- a/media/ffmpeg/ffmpeg_common.cc
+++ b/media/ffmpeg/ffmpeg_common.cc
@@ -14,6 +14,7 @@
 #include "media/base/decoder_buffer.h"
 #include "media/base/video_decoder_config.h"
 #include "media/base/video_util.h"
+#include "media/media_features.h"
 
 namespace media {
 
@@ -67,6 +68,12 @@ static AudioCodec CodecIDToAudioCodec(AVCodecID codec_id) {
   switch (codec_id) {
     case AV_CODEC_ID_AAC:
       return kCodecAAC;
+#if BUILDFLAG(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:
@@ -323,6 +330,18 @@ bool AVCodecContextToAudioDecoderConfig(const AVCodecContext* codec_context,
     sample_rate = 48000;
   }
 
+#if BUILDFLAG(ENABLE_AC3_EAC3_AUDIO_DEMUXING)
+  // For AC3/EAC3 we enable only demuxing, but not decoding, so FFmpeg does not
+  // fill |sample_fmt|.
+  if (codec == kCodecAC3 || codec == kCodecEAC3) {
+    // The spec for AC3/EAC3 audio is ETSI TS 102 366. According to sections
+    // F.3.1 and F.5.1 in that spec the sample_format for AC3/EAC3 must be 16.
+    sample_format = kSampleFormatS16;
+  }
+#else
+  DCHECK(codec != kCodecAC3 && codec != kCodecEAC3);
+#endif
+
   base::TimeDelta seek_preroll;
   if (codec_context->seek_preroll > 0) {
     seek_preroll = base::TimeDelta::FromMicroseconds(
@@ -352,8 +371,9 @@ bool AVCodecContextToAudioDecoderConfig(const AVCodecContext* codec_context,
                      is_encrypted,
                      seek_preroll,
                      codec_context->delay);
-
-  if (codec != kCodecOpus) {
+  if (codec != kCodecOpus && codec != kCodecAC3 && codec != kCodecEAC3) {

[ddorwin, 2016/01/08 18:11:02]

That leaves this as the only code in this file not protected. We should be
consistent either way, though at least this would be optimized out of release
builds.

[servolk, 2016/01/08 20:02:32]

Done (and converted to switch to avoid splitting the if condition with #if
buildflag(ac3) )

+    // Verify that AudioConfig.bits_per_channel was calculated correctly for
+    // codecs that have |sample_fmt| set by FFmpeg.
     DCHECK_EQ(av_get_bytes_per_sample(codec_context->sample_fmt) * 8,
               config->bits_per_channel());
   }