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

ios/chrome/browser/ui/authentication/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/signin_confirmation_more.png",
     "resources/signin_confirmation_more@2x.png",
     "resources/signin_confirmation_more@3x.png",
   ]
@@ skipping 64 matching lines @@
     "//url",
   ]
   libs = [
     "CoreGraphics.framework",
     "QuartzCore.framework",
     "UIKit.framework",
   ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "authentication_flow_unittest.mm",
     "re_signin_infobar_delegate_unittest.mm",
     "signed_in_accounts_view_controller_unittest.mm",
   ]
   deps = [
     ":authentication",
     "//base",
     "//base/test:test_support",
@@ skipping 33 matching lines @@
     "//ios/chrome/browser/ui/tools_menu",
     "//ios/chrome/test/app:test_support",
     "//ios/chrome/test/earl_grey:test_support",
     "//ios/public/provider/chrome/browser/signin:test_support",
     "//ios/testing:ios_test_support",
     "//ios/testing/earl_grey:earl_grey_support",
     "//ios/third_party/earl_grey",
   ]
   libs = [ "XCTest.framework" ]
 }