Make an enum class out of NetEqDecoder, and hide the neteq_decoders_ table

webrtc/modules/audio_coding/main/acm2/rent_a_codec.cc

Index: webrtc/modules/audio_coding/main/acm2/rent_a_codec.cc
diff --git a/webrtc/modules/audio_coding/main/acm2/rent_a_codec.cc b/webrtc/modules/audio_coding/main/acm2/rent_a_codec.cc
index 45542093e807d50610a7e9bdd8fdd2fd50ad2f67..42f0a4c7db0b61dadcc2c61ee28d6b1a7d36b6e1 100644
--- a/webrtc/modules/audio_coding/main/acm2/rent_a_codec.cc
+++ b/webrtc/modules/audio_coding/main/acm2/rent_a_codec.cc
@@ -55,5 +55,16 @@ rtc::ArrayView<const CodecInst> RentACodec::Database() {
                                          NumberOfCodecs());
 }
 
+rtc::Maybe<NetEqDecoder> RentACodec::NetEqDecoderFromCodecId(CodecId codec_id,
+                                                             int num_channels) {
+  rtc::Maybe<int> i = CodecIndexFromId(codec_id);
+  if (!i)
+    return rtc::Maybe<NetEqDecoder>();
+  const NetEqDecoder ned = ACMCodecDB::neteq_decoders_[*i];
+  return (ned == NetEqDecoder::kDecoderOpus && num_channels == 2)
+             ? NetEqDecoder::kDecoderOpus_2ch
+             : ned;
+}
+
 }  // namespace acm2
 }  // namespace webrtc