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

ios/chrome/browser/itunes_links/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("itunes_links") {
   configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "itunes_links_observer.h",
     "itunes_links_observer.mm",
   ]
   deps = [
     "//base",
     "//ios/chrome/browser",
     "//ios/web",
     "//url",
   ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "itunes_links_observer_unittest.mm",
   ]
   deps = [
     ":itunes_links",
     "//base",
     "//ios/chrome/browser",
     "//ios/web:test_support",
     "//testing/gtest",
     "//third_party/ocmock",
     "//url",
   ]
 }