Fix style guide issues in audio mixer.

webrtc/modules/audio_mixer/audio_mixer_defines.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 28 matching lines @@
   // classes are allowed to return the same AudioFrame pointer on
   // different calls. The pointer must stay valid until the next
   // mixing call or until this audio source is disconnected from the
   // mixer.
   virtual AudioFrameWithMuted GetAudioFrameWithMuted(int32_t id,
                                                      int sample_rate_hz) = 0;
 
   // Returns true if the participant was mixed this mix iteration.
   bool IsMixed() const;
 
-  NewMixHistory* _mixHistory;
+  NewMixHistory* mix_history_;
 
  protected:
   MixerAudioSource();
   virtual ~MixerAudioSource();
 };
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_AUDIO_MIXER_AUDIO_MIXER_DEFINES_H_