Remove ACMCodecDB::Codec, and make the rest of ACMCodecDB private

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 76dc7bbf47da462287408e03d61e0766e0ce71eb..2a6c37ff36851ac32c7770175c193420ff0011b0 100644
--- a/webrtc/modules/audio_coding/main/acm2/rent_a_codec.cc
+++ b/webrtc/modules/audio_coding/main/acm2/rent_a_codec.cc
@@ -29,6 +29,11 @@ rtc::Maybe<CodecInst> RentACodec::CodecInstById(CodecId codec_id) {
   return mi ? rtc::Maybe<CodecInst>(Database()[*mi]) : rtc::Maybe<CodecInst>();
 }
 
+rtc::Maybe<RentACodec::CodecId> RentACodec::CodecIdByInst(
+    const CodecInst& codec_inst) {
+  return CodecIdFromIndex(ACMCodecDB::CodecNumber(codec_inst));
+}
+
 rtc::Maybe<CodecInst> RentACodec::CodecInstByParams(const char* payload_name,
                                                     int sampling_freq_hz,
                                                     int channels) {