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

ios/chrome/browser/ui/first_run/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/FirstRun.xcassets/checkbox.imageset/Contents.json",
     "resources/FirstRun.xcassets/checkbox.imageset/checkbox.png",
     "resources/FirstRun.xcassets/checkbox.imageset/checkbox@2x.png",
     "resources/FirstRun.xcassets/checkbox.imageset/checkbox@2x~ipad.png",
@@ skipping 57 matching lines @@
     "//ios/third_party/material_roboto_font_loader_ios",
     "//ios/web",
     "//ui/base",
     "//ui/gfx",
     "//url",
   ]
   libs = [ "UIKit.framework" ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "first_run_util_unittest.mm",
     "welcome_to_chrome_view_controller_unittest.mm",
   ]
   deps = [
     ":first_run",
     "//base",
     "//components/metrics",
     "//components/prefs",
@@ skipping 29 matching lines @@
     "//ios/chrome/browser/signin",
     "//ios/chrome/browser/sync",
     "//ios/chrome/test/app:test_support",
     "//ios/chrome/test/earl_grey:test_support",
     "//ios/public/provider/chrome/browser/signin:test_support",
     "//ios/third_party/earl_grey",
     "//ui/base",
   ]
   libs = [ "XCTest.framework" ]
 }