Add Opus decode wrapper to media.

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 c79e4807faf66c09006ba6508e66fc5c3bfa0822..0aacc5288d1b22bd5256c3b00035034359937045 100644
--- a/media/ffmpeg/ffmpeg_common.cc
+++ b/media/ffmpeg/ffmpeg_common.cc
@@ -83,6 +83,8 @@ AudioCodec CodecIDToAudioCodec(CodecID codec_id) {
       return kCodecGSM_MS;
     case CODEC_ID_PCM_MULAW:
       return kCodecPCM_MULAW;
+    case CODEC_ID_OPUS:
+      return kCodecOpus;
     default:
       DVLOG(1) << "Unknown audio CodecID: " << codec_id;
   }
@@ -124,6 +126,8 @@ static CodecID AudioCodecToCodecID(AudioCodec audio_codec,
       return CODEC_ID_GSM_MS;
     case kCodecPCM_MULAW:
       return CODEC_ID_PCM_MULAW;
+    case kCodecOpus:
+      return CODEC_ID_OPUS;
     default:
       DVLOG(1) << "Unknown AudioCodec: " << audio_codec;
   }
@@ -230,6 +234,14 @@ void AVCodecContextToAudioDecoderConfig(
                                          codec_context->channels);
   int samples_per_second = codec_context->sample_rate;
 
+  if (codec == kCodecOpus) {
+    // TODO(tomfinegan): |sample_fmt| in |codec_context| is -1... because
+    // libopusdec.c isn't built into ffmpegsumo...? Maybe it's not *that* big
+    // a deal since libopus will produce either float or S16 samples, and
+    // |OpusAudioDecoder| is the only provider of Opus support.
+    bytes_per_channel = av_get_bytes_per_sample(AV_SAMPLE_FMT_S16);
+  }
+
   config->Initialize(codec,
                      bytes_per_channel << 3,
                      channel_layout,