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

ios/chrome/app/safe_mode/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.
 
 bundle_data("resources") {
   sources = [
     "resources/fatal_error.png",
     "resources/fatal_error@2x.png",
     "resources/fatal_error@3x.png",
   ]
@@ skipping 37 matching lines @@
     "//ios/chrome/app:app_internal",
     "//ios/chrome/app/strings",
     "//ios/chrome/browser/ui/main",
     "//ios/chrome/test/base",
     "//ios/chrome/test/earl_grey:test_support",
     "//ios/third_party/earl_grey",
   ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "safe_mode_coordinator_unittest.mm",
     "safe_mode_view_controller_unittest.mm",
   ]
   deps = [
     ":safe_mode",
     "//base",
     "//breakpad:client",
     "//ios/chrome/browser/crash_report",
     "//ios/chrome/test/base",
     "//ios/chrome/test/ocmock",
     "//testing/gtest",
     "//third_party/ocmock",
   ]
   libs = [ "UIKit.framework" ]
 }