third_party: Add the libsync library from Android

third_party/libsync/BUILD.gn

+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+config("libsync_config") {
+  include_dirs = [ "include" ]
+}
+
+source_set("libsync") {
+  sources = [
+    "include/linux/sw_sync.h",
+    "include/linux/sync.h",
+    "include/sw_sync.h",
+    "include/sync/sync.h",
+    "strlcpy.c",
+    "sync.c",
+  ]
+
+  configs -= [ "//build/config/compiler:chromium_code" ]
+  configs += [ "//build/config/compiler:no_chromium_code" ]
+
+  public_configs = [ ":libsync_config" ]
+}