[ObjC ARC] Converts ios/chrome/test/app:test_support to ARC.

ios/chrome/test/app/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("test_support") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "bookmarks_test_util.h",
     "bookmarks_test_util.mm",
     "chrome_test_util.h",
     "chrome_test_util.mm",
     "histogram_test_util.h",
     "histogram_test_util.mm",
     "history_test_util.h",
     "history_test_util.mm",
@@ skipping 58 matching lines @@
     "//ios/public/provider/chrome/browser",
     "//ios/public/provider/chrome/browser/signin:test_support",
     "//ios/testing:ios_test_support",
     "//ios/web",
     "//ios/web/public/test",
     "//net",
     "//net:test_support",
     "//url",
   ]
 }