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

ios/chrome/browser/ntp_tiles/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("ntp_tiles") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "ios_most_visited_sites_factory.cc",
     "ios_most_visited_sites_factory.h",
     "ios_popular_sites_factory.cc",
     "ios_popular_sites_factory.h",
     "most_visited_sites_observer_bridge.h",
     "most_visited_sites_observer_bridge.mm",
   ]
   deps = [
     "//base",
     "//components/history/core/browser",
     "//components/image_fetcher/core",
     "//components/image_fetcher/ios",
     "//components/keyed_service/core",
     "//components/ntp_tiles",
     "//components/ntp_tiles:json_unsafe_parser",
     "//ios/chrome/browser",
     "//ios/chrome/browser/browser_state",
     "//ios/chrome/browser/favicon",
     "//ios/chrome/browser/history",
     "//ios/chrome/browser/search_engines",
     "//ios/chrome/browser/suggestions",
     "//ios/web",
   ]
 }