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

ios/chrome/browser/ui/settings/utils/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("utils") {
   sources = [
     "content_setting_backed_boolean.h",
     "content_setting_backed_boolean.mm",
     "observable_boolean.h",
     "pref_backed_boolean.h",
@@ skipping 17 matching lines @@
   sources = [
     "fake_observable_boolean.h",
     "fake_observable_boolean.mm",
   ]
   deps = [
     ":utils",
   ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "content_setting_backed_boolean_unittest.mm",
     "pref_backed_boolean_unittest.mm",
   ]
   deps = [
     ":test_support",
     ":utils",
     "//base",
     "//components/content_settings/core/browser",
     "//components/content_settings/core/common",
     "//components/prefs",
     "//components/prefs:test_support",
     "//components/sync_preferences:test_support",
     "//ios/chrome/browser/browser_state:test_support",
     "//ios/chrome/browser/content_settings",
     "//ios/web:test_support",
     "//testing/gtest",
   ]
 }