[ObjC ARC] Converts components/signin/ios/browser:unit_tests to ARC.

components/signin/ios/browser/BUILD.gn

 # Copyright 2014 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("browser") {
   sources = [
     "account_consistency_service.h",
     "account_consistency_service.mm",
     "manage_accounts_delegate.h",
     "merge_session_observer_bridge.h",
@@ skipping 30 matching lines @@
   ]
 
   public_deps = [
     ":browser",
     "//base",
     "//google_apis:test_support",
   ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "account_consistency_service_unittest.mm",
     "profile_oauth2_token_service_ios_delegate_unittest.mm",
   ]
 
   deps = [
     ":test_support",
     "//components/prefs:test_support",
     "//components/signin/core/browser",
     "//components/signin/core/browser:test_support",
     "//components/signin/core/common",
     "//components/sync_preferences:test_support",
     "//ios/web",
     "//ios/web/public/test",
     "//ios/web/public/test/fakes",
     "//third_party/ocmock",
   ]
 }