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

ios/chrome/browser/ui/context_menu/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("context_menu") {
   configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "context_menu_coordinator.h",
     "context_menu_coordinator.mm",
   ]
   deps = [
     "//base",
     "//ios/chrome/browser",
     "//ios/chrome/browser/ui/alert_coordinator",
     "//ios/web",
     "//ui/base",
     "//ui/strings",
   ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "context_menu_coordinator_unittest.mm",
   ]
   deps = [
     ":context_menu",
     "//base",
     "//ios/web",
     "//testing/gtest",
   ]
 }