Split audio mixer into interface and implementation.

.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/api:audio_mixer_api",
   "//webrtc/api:rtc_stats_api",
   "//webrtc/modules/audio_coding:g711_test",
   "//webrtc/modules/audio_coding:g722_test",
   "//webrtc/modules/audio_coding:ilbc_test",
   "//webrtc/modules/audio_coding:isac",
   "//webrtc/modules/audio_coding:isac_api_test",
   "//webrtc/modules/audio_coding:isac_fix_test",
   "//webrtc/modules/audio_coding:isac_switch_samprate_test",
   "//webrtc/modules/audio_coding:isac_test",
   "//webrtc/modules/audio_coding:webrtc_opus_fec_test",
@@ skipping 26 matching lines @@
   "//build/gypi_to_gn.py",
   "//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",
 ]