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

ios/chrome/browser/ui/activity_services/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.
 
 bundle_data("assets") {
   sources = [
     "resources/activity_services_print.xcassets/Contents.json",
     "resources/activity_services_print.xcassets/activity_services_print.imageset/Contents.json",
     "resources/activity_services_print.xcassets/activity_services_print.imageset/activity_services_print-60@2x.png",
     "resources/activity_services_print.xcassets/activity_services_print.imageset/activity_services_print-60@3x.png",
@@ skipping 54 matching lines @@
     "//ui/base",
     "//url",
   ]
   libs = [
     "MobileCoreServices.framework",
     "UIKit.framework",
   ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "activity_service_controller_unittest.mm",
     "activity_type_util_unittest.mm",
     "chrome_activity_item_thumbnail_generator_unittest.mm",
   ]
   deps = [
     ":activity_services",
     "//base",
     "//base/test:test_support",
@@ skipping 23 matching lines @@
     "//ios/chrome/browser/ui:ui_internal",
     "//ios/chrome/test/app:test_support",
     "//ios/chrome/test/earl_grey:test_support",
     "//ios/third_party/material_components_ios",
     "//ios/web:earl_grey_test_support",
     "//ios/web:test_support",
     "//ui/base",
   ]
   libs = [ "XCTest.framework" ]
 }