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

ios/chrome/browser/ui/ntp/recent_tabs/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("recent_tabs") {
   sources = [
     "recent_tabs_bridges.h",
     "recent_tabs_bridges.mm",
     "recent_tabs_panel_controller.h",
     "recent_tabs_panel_controller.mm",
@@ skipping 31 matching lines @@
   ]
   public_deps = [
     "//components/sync_sessions",
   ]
   allow_circular_includes_from =
       [ "//ios/chrome/browser/ui/ntp/recent_tabs/views" ]
   libs = [ "UIKit.framework" ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "recent_tabs_panel_controller_unittest.mm",
   ]
   deps = [
     ":recent_tabs",
     "//base",
     "//components/browser_sync",
     "//components/browser_sync:test_support",
     "//components/signin/core/browser",
@@ skipping 20 matching lines @@
     "//ios/chrome/app/strings",
     "//ios/chrome/browser/ui",
     "//ios/chrome/browser/ui/tools_menu",
     "//ios/chrome/test/app:test_support",
     "//ios/chrome/test/earl_grey:test_support",
     "//ios/third_party/earl_grey",
     "//ios/web:test_support",
   ]
   libs = [ "XCTest.framework" ]
 }