[ObjC ARC] Converts components/reading_list/ios:ios to ARC.

components/reading_list/ios/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("ios") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "favicon_web_state_dispatcher.h",
     "reading_list_model_bridge_observer.h",
     "reading_list_model_bridge_observer.mm",
   ]
   deps = [
     "//base",
     "//components/reading_list/core",
   ]
 }