[ObjC ARC] Converts ios/public/provider/chrome/browser/omaha:omaha to ARC.

ios/public/provider/chrome/browser/omaha/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("omaha") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "omaha_service_provider.h",
     "omaha_service_provider.mm",
     "omaha_xml_writer.h",
   ]
   deps = [
     "//base",
     "//url",
   ]
 }
 
 source_set("test_support") {
   testonly = true
   sources = [
     "test_omaha_service_provider.h",
     "test_omaha_service_provider.mm",
   ]
   deps = [
     ":omaha",
   ]
 }