Fix style guide issues in audio mixer.

webrtc/modules/audio_mixer/audio_mixer.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.
  */
 
@@ skipping 10 matching lines @@
 class MixerAudioSource;
 
 class AudioMixer {
  public:
   static const int kMaximumAmountOfMixedAudioSources = 3;
   enum Frequency {
     kNbInHz = 8000,
     kWbInHz = 16000,
     kSwbInHz = 32000,
     kFbInHz = 48000,
-    kLowestPossible = -1,
     kDefaultFrequency = kWbInHz
   };
 
   // Factory method. Constructor disabled.
   static std::unique_ptr<AudioMixer> Create(int id);
   virtual ~AudioMixer() {}
 
   // Add/remove audio sources as candidates for mixing.
   virtual int32_t SetMixabilityStatus(MixerAudioSource* audio_source,
                                       bool mixable) = 0;
@@ skipping 21 matching lines @@
 
   // Output level functions for VoEVolumeControl. Return value
   // between 0 and 9 is returned by voe::AudioLevel.
   virtual int GetOutputAudioLevel() = 0;
 
   // Return value between 0 and 0x7fff is returned by voe::AudioLevel.
   virtual int GetOutputAudioLevelFullRange() = 0;
 
  protected:
   AudioMixer() {}
+
+ private:
+  RTC_DISALLOW_COPY_AND_ASSIGN(AudioMixer);
 };
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_AUDIO_MIXER_AUDIO_MIXER_H_