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

ios/chrome/app/spotlight/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("spotlight") {
   sources = [
     "actions_spotlight_manager.h",
     "actions_spotlight_manager.mm",
     "base_spotlight_manager.h",
     "base_spotlight_manager.mm",
@@ skipping 31 matching lines @@
     "//ui/base",
     "//url",
   ]
   libs = [
     "CoreSpotlight.framework",
     "UIKit.framework",
   ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "spotlight_manager_unittest.mm",
   ]
   deps = [
     ":spotlight",
     "//base",
     "//components/bookmarks/browser",
     "//components/bookmarks/test",
     "//components/favicon/core",
     "//components/favicon_base",
     "//ios/chrome/browser/bookmarks",
     "//ios/public/provider/chrome/browser",
     "//ios/public/provider/chrome/browser/spotlight",
     "//net",
     "//testing/gtest",
   ]
   libs = [ "CoreSpotlight.framework" ]
 }