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

ios/chrome/browser/prefs/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("prefs") {
   configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "ios_chrome_pref_model_associator_client.cc",
     "ios_chrome_pref_model_associator_client.h",
     "ios_chrome_pref_service_factory.cc",
     "ios_chrome_pref_service_factory.h",
     "pref_observer_bridge.h",
     "pref_observer_bridge.mm",
   ]
   deps = [
     "//base",
     "//components/content_settings/core/browser",
     "//components/prefs",
     "//components/proxy_config",
     "//components/search_engines",
     "//components/sync_preferences",
     "//ios/chrome/browser",
   ]
 }
 
 source_set("browser_prefs") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "browser_prefs.h",
     "browser_prefs.mm",
   ]
   deps = [
     "//components/autofill/core/browser",
     "//components/browsing_data/core",
     "//components/content_settings/core/browser",
     "//components/dom_distiller/core",
     "//components/flags_ui",
@@ skipping 28 matching lines @@
     "//ios/chrome/browser/metrics",
     "//ios/chrome/browser/net",
     "//ios/chrome/browser/physical_web",
     "//ios/chrome/browser/signin",
     "//ios/chrome/browser/ui/bookmarks",
     "//ios/chrome/browser/voice:prefs",
     "//ios/public/provider/chrome/browser",
     "//ui/base",
   ]
 }