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

media/base/audio_discard_helper.h

Index: media/base/audio_discard_helper.h
diff --git a/media/base/audio_discard_helper.h b/media/base/audio_discard_helper.h
index 544c4cfa2e28232ffa59de45856980c7a19c723e..c96bf7abcc426dc1b453ec1d17a421d74de67655 100644
--- a/media/base/audio_discard_helper.h
+++ b/media/base/audio_discard_helper.h
@@ -34,7 +34,9 @@ class MEDIA_EXPORT AudioDiscardHelper {
   // corresponding to the first encoded buffer is output.  These frames are not
   // represented in the encoded data stream and instead are an artifact of how
   // most MP3 decoders work.  See http://lame.sourceforge.net/tech-FAQ.txt
-  AudioDiscardHelper(int sample_rate, size_t decoder_delay);
+  AudioDiscardHelper(int sample_rate,
+                     size_t decoder_delay,
+                     bool delayed_discard);
   ~AudioDiscardHelper();
 
   // Converts a TimeDelta to a frame count based on the constructed sample rate.
@@ -92,7 +94,7 @@ class MEDIA_EXPORT AudioDiscardHelper {
   // decoded buffer.  In this case DiscardPadding must be carried over from the
   // previous encoded buffer.  Enabled automatically if an encoded buffer is
   // given to ProcessBuffers() with a NULL decoded buffer.
-  bool delayed_discard_;
+  const bool delayed_discard_;
   DecoderBuffer::DiscardPadding delayed_discard_padding_;
 
   // When |decoder_delay_| > 0, the number of frames which should be discarded