[ObjC ARC] Converts ios/chrome/browser/signin:test_support to ARC.

ios/chrome/browser/signin/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.
 
 source_set("signin") {
   sources = [
     "about_signin_internals_factory.cc",
     "about_signin_internals_factory.h",
     "account_consistency_service_factory.h",
     "account_consistency_service_factory.mm",
@@ skipping 59 matching lines @@
     "//ios/chrome/common",
     "//ios/public/provider/chrome/browser",
     "//ios/public/provider/chrome/browser/signin",
     "//ios/web",
     "//net",
     "//url",
   ]
 }
 
 source_set("test_support") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "authentication_service_fake.h",
     "authentication_service_fake.mm",
     "fake_oauth2_token_service_builder.h",
     "fake_oauth2_token_service_builder.mm",
     "fake_signin_manager_builder.cc",
     "fake_signin_manager_builder.h",
   ]
   deps = [
@@ skipping 52 matching lines @@
     "signin_capability.cc",
     "signin_capability.h",
   ]
   deps = [
     ":signin",
     "//base",
     "//components/signin/core/browser",
     "//ios/chrome/browser/browser_state",
   ]
 }