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

ios/chrome/browser/signin/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("signin") {
   sources = [
     "about_signin_internals_factory.cc",
     "about_signin_internals_factory.h",
     "account_consistency_service_factory.h",
     "account_consistency_service_factory.mm",
@@ skipping 81 matching lines @@
     "//components/signin/core/browser:test_support",
     "//components/signin/ios/browser",
     "//ios/chrome/browser/browser_state",
     "//ios/chrome/browser/sync",
     "//ios/public/provider/chrome/browser",
     "//ios/public/provider/chrome/browser/signin",
   ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "authentication_service_unittest.mm",
     "chrome_identity_service_observer_bridge_unittest.mm",
     "gaia_auth_fetcher_ios_unittest.mm",
   ]
   deps = [
     ":signin",
     ":test_support",
     "//base",
@@ skipping 30 matching lines @@
     "signin_capability.cc",
     "signin_capability.h",
   ]
   deps = [
     ":signin",
     "//base",
     "//components/signin/core/browser",
     "//ios/chrome/browser/browser_state",
   ]
 }