Switch to using new ACM methods for encoder management

webrtc/modules/audio_coding/acm2/rent_a_codec.h

 /*
  *  Copyright (c) 2015 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 194 matching lines @@
     ACMVADMode vad_mode = VADNormal;
 
     // Maps from RTP timestamp rate (in Hz) to payload type.
     std::map<int, int> cng_payload_types;
     std::map<int, int> red_payload_types;
   };
 
   // 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.
   std::unique_ptr<AudioDecoder> RentIsacDecoder();
 
  private:
   std::unique_ptr<AudioEncoder> speech_encoder_;
   std::unique_ptr<AudioEncoder> cng_encoder_;
   std::unique_ptr<AudioEncoder> red_encoder_;
   rtc::scoped_refptr<LockedIsacBandwidthInfo> isac_bandwidth_info_;
 
   RTC_DISALLOW_COPY_AND_ASSIGN(RentACodec);
 };
 
 }  // namespace acm2
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_AUDIO_CODING_ACM2_RENT_A_CODEC_H_