GN: New conventions, default target and refactorings

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")
 
+group("audio_mixer") {
+  public_deps = [
+    ":audio_frame_manipulator",
+    ":audio_mixer_impl",
+  ]
+}
+
 rtc_static_library("audio_mixer_impl") {
   visibility = [
     "../../audio:audio",
     "../../modules/*",
   ]
   sources = [
     "audio_mixer_impl.cc",
     "audio_mixer_impl.h",
   ]
 
@@ skipping 24 matching lines @@
   sources = [
     "audio_frame_manipulator.cc",
     "audio_frame_manipulator.h",
   ]
 
   deps = [
     "../../base:rtc_base_approved",
     "../../modules/utility",
   ]
 }