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

NOTRY=True

Review-Url: https://codereview.webrtc.org/2228503003
Cr-Commit-Position: refs/heads/master@{#13677}
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 9d813b1..00bcc3e 100755
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -20,6 +20,7 @@
   'webrtc/call',
   'webrtc/common_video',
   'webrtc/examples',
+  'webrtc/modules/audio_mixer',
   'webrtc/modules/bitrate_controller',
   'webrtc/modules/congestion_controller',
   'webrtc/modules/pacing',