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

ios/chrome/browser/ui/settings/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("resources") {
   sources = [
     "resources/app_icon_placeholder.png",
     "resources/app_icon_placeholder@2x.png",
     "resources/app_icon_placeholder@3x.png",
     "resources/encryption_error.png",
@@ skipping 216 matching lines @@
     "//ios/chrome/browser/sync",
     "//ios/chrome/browser/sync:test_support",
     "//ios/chrome/browser/ui/collection_view:test_support",
     "//ios/public/provider/chrome/browser/signin:test_support",
     "//ios/web:test_support",
     "//testing/gtest",
   ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "about_chrome_collection_view_controller_unittest.mm",
     "autofill_collection_view_controller_unittest.mm",
     "autofill_profile_edit_collection_view_controller_unittest.mm",
     "bandwidth_management_collection_view_controller_unittest.mm",
     "block_popups_collection_view_controller_unittest.mm",
     "clear_browsing_data_collection_view_controller_unittest.mm",
     "content_settings_collection_view_controller_unittest.mm",
     "contextual_search_collection_view_controller_unittest.mm",
@@ skipping 125 matching lines @@
     "//ios/web:test_support",
     "//net",
     "//ui/base",
     "//url",
   ]
   libs = [
     "UIKit.framework",
     "XCTest.framework",
   ]
 }