[ObjC ARC] Converts ios/chrome/app:main to ARC.

ios/chrome/app/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.
 
 import("//build/config/ios/rules.gni")
 import("//build/config/mac/base_rules.gni")
 import("//build/mac/tweak_info_plist.gni")
 import("//ios/build/chrome_build.gni")
 import("//ios/public/provider/chrome/browser/build_config.gni")
 
@@ skipping 224 matching lines @@
   libs = [
     "CoreSpotlight.framework",
     "MediaPlayer.framework",
   ]
 
   allow_circular_includes_from =
       [ "//ios/chrome/app/application_delegate:application_delegate_internal" ]
 }
 
 source_set("main") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "chrome_exe_main.mm",
   ]
 
   deps = [
     ":app_internal",
     "//base",
     "//components/crash/core/common",
     "//ios/chrome/app/startup:startup_basic",
     "//ios/chrome/browser:browser_internal",
@@ skipping 81 matching lines @@
       ]
       sources = [
         "$root_out_dir/share_extension.appex",
       ]
       outputs = [
         "{{bundle_plugins_dir}}/{{source_file_part}}",
       ]
     }
   }
 }