Use ffmpeg for opus decoding, no need to maintain our decoder.

media/filters/ffmpeg_audio_decoder.cc

Index: media/filters/ffmpeg_audio_decoder.cc
diff --git a/media/filters/ffmpeg_audio_decoder.cc b/media/filters/ffmpeg_audio_decoder.cc
index 9f5325a24803ed92dc72f67d9a81329c0a6ffc29..602f5e0f24554090da6013a9fcd6f418ba5fe88e 100644
--- a/media/filters/ffmpeg_audio_decoder.cc
+++ b/media/filters/ffmpeg_audio_decoder.cc
@@ -403,6 +403,9 @@ bool FFmpegAudioDecoder::ConfigureDecoder() {
   codec_context_->get_buffer2 = GetAudioBuffer;
   codec_context_->refcounted_frames = 1;
 
+  if (config_.codec() == kCodecOpus)
+    codec_context_->request_sample_fmt = AV_SAMPLE_FMT_FLT;
+
   AVCodec* codec = avcodec_find_decoder(codec_context_->codec_id);
   if (!codec || avcodec_open2(codec_context_.get(), codec, NULL) < 0) {
     DLOG(ERROR) << "Could not initialize audio decoder: "
@@ -432,9 +435,13 @@ bool FFmpegAudioDecoder::ConfigureDecoder() {
 }
 
 void FFmpegAudioDecoder::ResetTimestampState() {
-  discard_helper_.reset(new AudioDiscardHelper(config_.samples_per_second(),
-                                               config_.codec_delay()));
-  discard_helper_->Reset(config_.codec_delay());
+  // Opus codec delay is handled by ffmpeg.
+  const int codec_delay =
+      config_.codec() == kCodecOpus ? 0 : config_.codec_delay();
+  discard_helper_.reset(
+      new AudioDiscardHelper(config_.samples_per_second(), codec_delay,
+                             config_.codec() == kCodecVorbis));
+  discard_helper_->Reset(codec_delay);
 }
 
 }  // namespace media