Split audio mixer into interface and implementation.

webrtc/modules/audio_mixer/BUILD.gn

 # Copyright (c) 2014 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("../../build/webrtc.gni")
 
-config("audio_conference_mixer_config") {
-  include_dirs = [ "../../modules/include" ]
-}
-
-rtc_static_library("audio_mixer") {
+rtc_static_library("audio_mixer_impl") {
+  visibility = [
+    "../../modules/*",

[the sun, 2016/10/12 15:26:10]

Does this mean it is visible to webrtc/modules/? Why is that necessary?

[aleloi, 2016/10/12 15:36:02]

The default visibility is that everything is visible everywhere. Since this is
an interior implementation, I wanted to forbid depending on it. The current plan
is to create a mixer in audio_state (which is in //webrtc/audio:audio). A mixer
is also created in //webrtc/modules:modules_unittests. Additionally, there is a
big target that contains everything, //:root and //webrtc:webrtc. It also
contains the mixer. This target is named differently for different platforms,
and setting visibility to just //webrtc/modules:modules didn't compile.

Therefore, I set visibility to modules/*.

[the sun, 2016/10/13 07:55:00]

Ah, I forgot about the unit test. It would be nice indeed if the unit test
binary didn't need to pull in the .cc put could depend on a component-local lib
containing the test, but perhaps that's not possible with gtest. :/

+    "../../audio:audio",
+  ]
   sources = [
     "audio_frame_manipulator.cc",
     "audio_frame_manipulator.h",
-    "audio_mixer.h",
     "audio_mixer_impl.cc",
     "audio_mixer_impl.h",
     "audio_source_with_mix_status.cc",
     "audio_source_with_mix_status.h",
   ]
 
   public = [
-    "audio_mixer.h",
+    "audio_mixer_impl.h",
   ]
 
-  public_configs = [ ":audio_conference_mixer_config" ]
-
   deps = [
     "../..:webrtc_common",
+    "../../api:audio_mixer_api",
     "../../base:rtc_base_approved",
     "../../modules/audio_processing",
     "../../modules/utility",
     "../../system_wrappers",
-    "../../voice_engine:level_indicator",
   ]
 }