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

ios/chrome/browser/metrics/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("metrics") {
   configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "field_trial_synchronizer.cc",
     "field_trial_synchronizer.h",
     "ios_chrome_metrics_service_accessor.cc",
@@ skipping 111 matching lines @@
   ]
   deps = [
     "//base",
     "//ios/chrome/browser/tabs",
     "//ios/chrome/browser/web_state_list",
   ]
   configs += [ "//build/config/compiler:enable_arc" ]
 }
 
 source_set("unit_tests_internal") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "first_user_action_recorder_unittest.cc",
     "size_class_recorder_unittest.mm",
     "tab_usage_recorder_unittest.mm",
   ]
   deps = [
     ":metrics",
     ":metrics_internal",
     "//base",
@@ skipping 65 matching lines @@
     "//ios/chrome/app/strings",
     "//ios/chrome/browser/ui",
     "//ios/chrome/browser/ui/tools_menu",
     "//ios/chrome/test/app:test_support",
     "//ios/chrome/test/earl_grey:test_support",
     "//ios/testing:ios_test_support",
     "//ui/base",
   ]
   configs += [ "//build/config/compiler:enable_arc" ]
 }