Add GN Build file for rtc_sound target.

Tested on Linux with the following command lines:

$ gn gen out-gn/Release --args='is_debug=false target_cpu="x64"
build_with_chromium=false'
$ ninja -C out-gn/Release rtc_sound

BUG=webrtc:4160
R=kjellander@webrtc.org

Review URL: https://codereview.webrtc.org/1425583002

Cr-Commit-Position: refs/heads/master@{#10405}
diff --git a/webrtc/BUILD.gn b/webrtc/BUILD.gn
index ac14d7d..c8b7ea0 100644
--- a/webrtc/BUILD.gn
+++ b/webrtc/BUILD.gn
@@ -178,8 +178,8 @@
   public_configs = [ ":common_inherited_config" ]
 
   deps = [
-    "audio",
     ":webrtc_common",
+    "audio",
     "base:rtc_base",
     "call",
     "common_audio",
@@ -195,6 +195,7 @@
     "modules/utility",
     "modules/video_coding",
     "modules/video_processing",
+    "sound:rtc_sound",
     "system_wrappers",
     "tools",
     "video",
diff --git a/webrtc/sound/BUILD.gn b/webrtc/sound/BUILD.gn
new file mode 100644
index 0000000..82bfdd1
--- /dev/null
+++ b/webrtc/sound/BUILD.gn
@@ -0,0 +1,55 @@
+# Copyright 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.
+import("../build/webrtc.gni")
+
+static_library("rtc_sound") {
+  sources = [
+    "automaticallychosensoundsystem.h",
+    "nullsoundsystem.cc",
+    "nullsoundsystem.h",
+    "nullsoundsystemfactory.cc",
+    "nullsoundsystemfactory.h",
+    "platformsoundsystem.cc",
+    "platformsoundsystem.h",
+    "platformsoundsystemfactory.cc",
+    "platformsoundsystemfactory.h",
+    "sounddevicelocator.h",
+    "soundinputstreaminterface.h",
+    "soundoutputstreaminterface.h",
+    "soundsystemfactory.h",
+    "soundsysteminterface.cc",
+    "soundsysteminterface.h",
+    "soundsystemproxy.cc",
+    "soundsystemproxy.h",
+  ]
+
+  if (is_clang) {
+    # Suppress warnings from Chrome's Clang plugins.
+    # See http://code.google.com/p/webrtc/issues/detail?id=163 for details.
+    configs -= [ "//build/config/clang:find_bad_constructs" ]
+  }
+
+  deps = [
+    "//webrtc/base:rtc_base",
+  ]
+
+  if (is_linux) {
+    sources += [
+      "alsasoundsystem.cc",
+      "alsasoundsystem.h",
+      "alsasymboltable.cc",
+      "alsasymboltable.h",
+      "linuxsoundsystem.cc",
+      "linuxsoundsystem.h",
+      "pulseaudiosoundsystem.cc",
+      "pulseaudiosoundsystem.h",
+      "pulseaudiosymboltable.cc",
+      "pulseaudiosymboltable.h",
+    ]
+  }
+}