[ObjC ARC] Converts ios/chrome/browser/ui/downloads:unit_tests 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 47 matching lines @@
     "//ios/web",
     "//ios/web:core",
     "//net",
     "//ui/base",
     "//ui/gfx",
   ]
   libs = [ "UIKit.framework" ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "download_manager_controller_unittest.mm",
   ]
   deps = [
     ":downloads",
     "//base",
     "//ios/chrome/browser",
     "//ios/web:test_support",
     "//net:test_support",
     "//testing/gtest",
   ]
 }