Add installer::SetupSingleton.

chrome/installer/setup/BUILD.gn

 # Copyright 2015 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.
 
 import("//chrome/version.gni")
 import("//testing/test.gni")
 
 if (is_win) {
   executable("setup") {
     sources = [
@@ skipping 32 matching lines @@
       "install_worker.cc",
       "install_worker.h",
       "installer_crash_reporter_client.cc",
       "installer_crash_reporter_client.h",
       "installer_crash_reporting.cc",
       "installer_crash_reporting.h",
       "installer_metrics.cc",
       "installer_metrics.h",
       "setup_constants.cc",
       "setup_constants.h",
+      "setup_singleton.cc",
+      "setup_singleton.h",
       "setup_util.cc",
       "setup_util.h",
       "update_active_setup_version_work_item.cc",
       "update_active_setup_version_work_item.h",
       "user_hive_visitor.cc",
       "user_hive_visitor.h",
     ]
 
     public_deps = [
       "//base",
@@ skipping 19 matching lines @@
   }
 
   # This test covers the mini installer and setup tests.
   test("setup_unittests") {
     sources = [
       "archive_patch_helper_unittest.cc",
       "install_unittest.cc",
       "install_worker_unittest.cc",
       "memory_unittest.cc",
       "run_all_unittests.cc",
+      "setup_singleton_unittest.cc",
       "setup_util_unittest.cc",
       "setup_util_unittest.h",
       "update_active_setup_version_work_item_unittest.cc",
       "user_hive_visitor_unittest.cc",
     ]
 
     deps = [
       ":lib",
       "//base",
       "//base:i18n",
       "//base/test:test_support",
       "//chrome/install_static:install_static_util",
       "//chrome/installer/mini_installer:unit_tests",
       "//testing/gmock",
       "//testing/gtest",
     ]
 
     data = [
       "//chrome/installer/test/data/",
       "//chrome/test/data/installer/",
     ]
   }
 }