Relanding https://codereview.webrtc.org/1715883002/ in pieces.

webrtc/voice_engine/dtmf_inband.h

 /*
  *  Copyright (c) 2011 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.
  */
 
 #ifndef WEBRTC_VOICE_ENGINE_DTMF_INBAND_H_
 #define WEBRTC_VOICE_ENGINE_DTMF_INBAND_H_
 
 #include "webrtc/typedefs.h"
 #include "webrtc/voice_engine/voice_engine_defines.h"
 #include "webrtc/base/criticalsection.h"
 
 namespace webrtc {
 
+// TODO(solenberg): Used as a DTMF tone generator in voe::OutputMixer. Pull out
+//                  the one in NetEq and use that instead? We don't need several
+//                  implemenations of this.
 class DtmfInband
 {
 public:
     DtmfInband(int32_t id);
 
     virtual ~DtmfInband();
 
     void Init();
 
     int SetSampleRate(uint16_t frequency);
@@ skipping 48 matching lines @@
     int16_t _attenuationDb;       // [0, 36]
     int32_t _lengthMs;
     bool _reinit;  // 'true' if the oscillator should be reinit for next event
     bool _playing;
     uint32_t _delaySinceLastToneMS; // time since last generated tone [ms]
 };
 
 }  // namespace webrtc
 
 #endif // #ifndef WEBRTC_VOICE_ENGINE_DTMF_INBAND_H_