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

ios/chrome/browser/upgrade/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("upgrade") {
   sources = [
     "upgrade_center.h",
     "upgrade_center.mm",
     "upgrade_recommended_details.h",
   ]
   deps = [
     "//base",
     "//components/infobars/core",
     "//components/version_info",
     "//ios/chrome/app/strings",
     "//ios/chrome/browser",
     "//ios/chrome/browser/ui/commands",
     "//ios/chrome/browser/ui/infobars:resources",
     "//ios/web",
     "//net",
     "//ui/base",
     "//ui/gfx",
     "//url",
   ]
   libs = [ "UIKit.framework" ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "upgrade_center_unittest.mm",
   ]
   deps = [
     ":upgrade",
     "//base",
     "//testing/gtest",
   ]
 }