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

ios/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") {
   sources = [
     "omaha_service.h",
     "omaha_service.mm",
   ]
   deps = [
@@ skipping 12 matching lines @@
     "//ios/public/provider/chrome/browser",
     "//ios/public/provider/chrome/browser/omaha",
     "//ios/web",
     "//net",
     "//third_party/libxml",
     "//url",
   ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "omaha_service_unittest.mm",
   ]
   deps = [
     ":omaha",
     "//base",
     "//components/metrics",
     "//components/prefs",
     "//components/version_info",
     "//ios/chrome/browser",
     "//ios/chrome/browser/browser_state:test_support",
     "//ios/chrome/common",
     "//ios/chrome/test:test_support",
     "//ios/public/provider/chrome/browser",
     "//ios/public/provider/chrome/browser/omaha",
     "//ios/web",
     "//ios/web:test_support",
     "//net:test_support",
     "//testing/gtest",
   ]
 }