Added new mixer folder to CPPLINT_DIRS. Fixed one small issue.

PRESUBMIT.py

 # Copyright (c) 2012 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.
 
 import json
 import os
 import platform
 import re
 import subprocess
 import sys
 
 
 # Directories that will be scanned by cpplint by the presubmit script.
 CPPLINT_DIRS = [
   'webrtc/audio',
   'webrtc/call',
   'webrtc/common_video',
   'webrtc/examples',
+  'webrtc/modules/audio_mixer',
   'webrtc/modules/bitrate_controller',
   'webrtc/modules/congestion_controller',
   'webrtc/modules/pacing',
   'webrtc/modules/remote_bitrate_estimator',
   'webrtc/modules/rtp_rtcp',
   'webrtc/modules/video_coding',
   'webrtc/modules/video_processing',
   'webrtc/tools',
   'webrtc/video',
 ]
@@ skipping 504 matching lines @@
     for builder in masters[master]:
       if 'presubmit' in builder:
         # Do not trigger presubmit builders, since they're likely to fail
         # (e.g. OWNERS checks before finished code review), and we're running
         # local presubmit anyway.
         pass
       else:
         try_config[master][builder] = ['defaulttests']
 
   return try_config