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

ios/chrome/browser/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("//build/buildflag_header.gni")
 import("//build/config/features.gni")
 import("//build/config/ios/rules.gni")
 import("//rlz/features/features.gni")
 import("//third_party/protobuf/proto_library.gni")
 
@@ skipping 201 matching lines @@
   sources = [
     "resources/Settings.bundle/Experimental.plist",
     "resources/Settings.bundle/Root.plist",
   ]
   outputs = [
     "{{bundle_resources_dir}}/Settings.bundle/{{source_file_part}}",
   ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "callback_counter_unittest.mm",
     "chrome_url_util_unittest.mm",
     "crash_loop_detection_util_unittest.mm",
     "install_time_util_unittest.mm",
     "installation_notifier_unittest.mm",
     "ios_chrome_io_thread_unittest.mm",
     "notification_promo_unittest.cc",
   ]
@@ skipping 17 matching lines @@
 
 source_set("browser_internal") {
   sources = [
     "callback_counter.h",
     "callback_counter.mm",
   ]
   deps = [
     "//base",
   ]
 }