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

.gn

 # Copyright (c) 2015 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.
 
 # The location of the build configuration file.
 buildconfig = "//build/config/BUILDCONFIG.gn"
 
 # The secondary source root is a parallel directory tree where
 # GN build files are placed when they can not be placed directly
 # in the source tree, e.g. for third party source trees.
 secondary_source = "//build/secondary/"
 
 # These are the targets to check headers for by default. The files in targets
 # matching these patterns (see "gn help label_pattern" for format) will have
 # their includes checked for proper dependencies when you run either
 # "gn check" or "gn gen --check".
 # TODO(kjellander): Keep adding paths to this list as work in webrtc:5589 is done.
-check_targets = [ "//webrtc/voice_engine:level_indicator" ]
+check_targets = [
+  "//webrtc/voice_engine:level_indicator",
+  "//webrtc/modules/audio_mixer:audio_conference_mixer",
+]
 
 # These are the list of GN files that run exec_script. This whitelist exists
 # to force additional review for new uses of exec_script, which is strongly
 # discouraged except for gypi_to_gn calls.
 exec_script_whitelist = [
   "//build/config/BUILD.gn",
   "//build/config/android/BUILD.gn",
   "//build/config/android/config.gni",
   "//build/config/android/internal_rules.gni",
   "//build/config/android/rules.gni",
@@ skipping 11 matching lines @@
   "//build/toolchain/concurrent_links.gni",
   "//build/toolchain/gcc_toolchain.gni",
   "//build/toolchain/mac/BUILD.gn",
   "//build/toolchain/win/BUILD.gn",
   "//third_party/boringssl/BUILD.gn",
   "//third_party/openh264/BUILD.gn",
   "//third_party/opus/BUILD.gn",
   "//webrtc/modules/video_render/BUILD.gn",
   "//webrtc/modules/video_render/BUILD.gn",
 ]