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

ios/chrome/app/startup/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.
 
 import("//ios/public/provider/chrome/browser/build_config.gni")
 
 source_set("startup") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "background_upload_alert.h",
     "background_upload_alert.mm",
     "chrome_main_starter.h",
     "chrome_main_starter.mm",
     "client_registration.h",
     "client_registration.mm",
     "ios_chrome_main.h",
     "ios_chrome_main.mm",
     "ios_chrome_main_delegate.h",
@@ skipping 16 matching lines @@
     "//ios/chrome/browser/web",
     "//ios/chrome/browser/web:web_internal",
     "//ios/net",
     "//ios/public/provider/chrome/browser",
     "//ios/web",
     "//ios/web/public/app",
     "//skia",
     ios_provider_target,
   ]
 }