Added webrtc/audio/utility directory and empty GN target.

This Cl is the first step of a 3-step process to trick the build system of a
webrtc dependency into not failing.

The second step will be to register this folder in the dependency
build system. It cannot be done first, because there must be a GN
dependency from some existing target to a target in the new folder.

The third step is to land https://codereview.webrtc.org/2424173003, in
which the webrtc/audio/utility is used.

When a new folder with a publicly visible header is added, the build
files of said build system must be manually updated to register the
new folder.

BUG=webrtc:6548

Review-Url: https://codereview.webrtc.org/2532403003
Cr-Commit-Position: refs/heads/master@{#15299}

webrtc/audio/BUILD.gn

@@ -37,6 +37,11 @@ rtc_static_library("audio") {
     "../modules/audio_processing",
     "../system_wrappers",
     "../voice_engine",
+    "utility",  # Bogus dep, needed for landing
+                # codereview.webrtc.org/2424173003 without breaking
+                # internal projects. See
+                # bugs.webrtc.org/6548. TODO(aleloi): remove dependency
+                # when codereview.webrtc.org/2424173003 has landed.
   ]
 }
 if (rtc_include_tests) {

webrtc/audio/utility/BUILD.gn

@@ -0,0 +1,11 @@
+# Copyright (c) 2016 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("utility") {
+}