Add support for external decoders in ACM

webrtc/modules/audio_coding/main/acm2/audio_coding_module_impl.h

Index: webrtc/modules/audio_coding/main/acm2/audio_coding_module_impl.h
diff --git a/webrtc/modules/audio_coding/main/acm2/audio_coding_module_impl.h b/webrtc/modules/audio_coding/main/acm2/audio_coding_module_impl.h
index 19ca01bf9ed43c8442bbbb76c845922cc8d0c6b2..beb49bc4ce5bcf5a8cc3b331b63eb600cb20716e 100644
--- a/webrtc/modules/audio_coding/main/acm2/audio_coding_module_impl.h
+++ b/webrtc/modules/audio_coding/main/acm2/audio_coding_module_impl.h
@@ -137,6 +137,11 @@ class AudioCodingModuleImpl : public AudioCodingModule {
   // for codecs, CNG, DTMF, RED.
   int RegisterReceiveCodec(const CodecInst& receive_codec) override;
 
+  int RegisterExternalReceiveCodec(int rtp_payload_type,
+                                   AudioDecoder* external_decoder,
+                                   int sample_rate_hz,
+                                   int num_channels) override;
+
   // Get current received codec.
   int ReceiveCodec(CodecInst* current_codec) const override;