Added new mixer to |check_targets| in .gn and fixed include/depend errors.

Also fixed one small chromium-style error in the new mixer.

NOTRY=True

Committed: https://crrev.com/d700bef583d29ba2834ae57b3af7e8d3b8306cb9
Review-Url: https://codereview.webrtc.org/2234293002
Cr-Original-Commit-Position: refs/heads/master@{#13752}
Cr-Commit-Position: refs/heads/master@{#13772}
diff --git a/.gn b/.gn
index 841989c..4cd7b01 100644
--- a/.gn
+++ b/.gn
@@ -22,6 +22,7 @@
 check_targets = [
   "//webrtc/voice_engine:level_indicator",
   "//webrtc/modules/audio_coding:isac_fix_test",
+  "//webrtc/modules/audio_mixer:audio_conference_mixer",
 ]
 
 # These are the list of GN files that run exec_script. This whitelist exists