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

ios/chrome/browser/ui/downloads/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.
 
 import("//build/config/ios/rules.gni")
 
 bundle_data("assets") {
   sources = [
     "resources/download_manager_assets.xcassets/Contents.json",
     "resources/download_manager_assets.xcassets/error_icon.imageset/Contents.json",
@@ skipping 16 matching lines @@
   outputs = [
     "{{bundle_resources_dir}}/{{source_file_part}}",
   ]
 }
 
 bundle_data_ib_file("download_manager_controller_xib") {
   source = "resources/DownloadManagerController.xib"
 }
 
 source_set("downloads") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "download_manager_controller.h",
     "download_manager_controller.mm",
   ]
   deps = [
     ":assets",
     ":download_manager_controller_xib",
     "//base",
     "//components/strings",
     "//ios/chrome/app/strings",
@@ skipping 29 matching lines @@
     "//base",
     "//ios/chrome/browser",
     "//ios/chrome/browser/store_kit",
     "//ios/chrome/browser/web:test_support",
     "//ios/web:test_support",
     "//net:test_support",
     "//testing/gtest",
     "//third_party/ocmock:ocmock",
   ]
 }