Switch to using new ACM methods for encoder management

webrtc/modules/audio_coding/acm2/rent_a_codec.h

Index: webrtc/modules/audio_coding/acm2/rent_a_codec.h
diff --git a/webrtc/modules/audio_coding/acm2/rent_a_codec.h b/webrtc/modules/audio_coding/acm2/rent_a_codec.h
index 2c5873d616fec138058cdcc86b47d339a832f1d9..a4026acd2832ff2e0b45e578bbb1ef4e1f391ddb 100644
--- a/webrtc/modules/audio_coding/acm2/rent_a_codec.h
+++ b/webrtc/modules/audio_coding/acm2/rent_a_codec.h
@@ -212,7 +212,7 @@ class RentACodec {
   // Creates and returns an audio encoder stack constructed to the given
   // specification. If the specification isn't compatible with the encoder, it
   // will be changed to match (things will be switched off). The speech encoder
-  // will be stolen.
+  // will be stolen. If the specification isn't complete, returns nullptr.
   std::unique_ptr<AudioEncoder> RentEncoderStack(StackParameters* param);
 
   // Creates and returns an iSAC decoder.