AGC2 as a new APM sub-module operating with hard-coded gain.

webrtc/modules/audio_processing/agc2/BUILD.gn

+# Copyright (c) 2017 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("../../../webrtc.gni")
+
+rtc_static_library("agc2") {
+  visibility = [ "../:*" ]  # Only APM targets can depend on this.
+  sources = [
+    "gain_controller2.cc",
+    "gain_controller2.h",
+  ]
+}
+
+rtc_source_set("unittests") {
+  testonly = true
+  visibility = [ "../:*" ]  # Only APM targets can depend on this.
+  sources = [
+    "gain_control2_unittest.cc",
+  ]
+  deps = [
+    "//testing/gtest",
+  ]
+}