[ObjC ARC] Converts ios/chrome/browser/ui/promos:unit_tests to ARC.

ios/chrome/browser/ui/promos/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.
 
 source_set("promos") {
   sources = [
     "promo_view_controller.h",
     "signin_promo_view_controller.h",
     "signin_promo_view_controller.mm",
   ]
   deps = [
     "//base",
     "//components/signin/core/browser",
     "//components/version_info",
     "//ios/chrome/app:tests_hook",
     "//ios/chrome/app/strings",
     "//ios/chrome/browser/browser_state",
     "//ios/chrome/browser/signin",
     "//ios/chrome/browser/ui/authentication",
     "//ios/chrome/browser/ui/commands",
     "//ios/public/provider/chrome/browser",
     "//ios/public/provider/chrome/browser/signin",
     "//net",
   ]
   libs = [ "UIKit.framework" ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "signin_promo_view_controller_unittest.mm",
   ]
   deps = [
     ":promos",
     "//base",
     "//components/pref_registry",
     "//components/sync_preferences",
     "//components/sync_preferences:test_support",
     "//ios/chrome/browser/browser_state:test_support",
     "//ios/chrome/browser/prefs:browser_prefs",
     "//ios/chrome/browser/signin",
     "//ios/chrome/browser/signin:test_support",
     "//ios/chrome/test:test_support",
     "//ios/public/provider/chrome/browser/signin:test_support",
     "//ios/web:test_support",
     "//testing/gtest",
     "//third_party/ocmock",
   ]
 }