[ObjC ARC] Converts ios/chrome/app:app 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")
 
 source_set("app") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "deferred_initialization_runner.h",
     "deferred_initialization_runner.mm",
     "safe_mode_crashing_modules_config.h",
     "safe_mode_crashing_modules_config.mm",
     "safe_mode_util.cc",
     "safe_mode_util.h",
   ]
 
   deps = [
@@ skipping 322 matching lines @@
       ]
       sources = [
         "$root_out_dir/share_extension.appex",
       ]
       outputs = [
         "{{bundle_plugins_dir}}/{{source_file_part}}",
       ]
     }
   }
 }