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

components/dom_distiller/ios/BUILD.gn

 # Copyright 2015 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 = [
     "distiller_page_factory_ios.h",
     "distiller_page_factory_ios.mm",
     "distiller_page_ios.h",
     "distiller_page_ios.mm",
   ]
 
   deps = [
     "//base",
     "//components/dom_distiller/core",
     "//components/dom_distiller/core/proto",
     "//components/favicon/ios",
     "//ios/web",
     "//url",
   ]
 }