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

ios/chrome/browser/ui/main/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("main") {
   sources = [
     "browser_view_information.h",
     "browser_view_wrangler.h",
     "browser_view_wrangler.mm",
     "main_coordinator.h",
@@ skipping 15 matching lines @@
     "//ios/chrome/browser/tabs:tabs_internal",
     "//ios/public/provider/chrome/browser",
   ]
   public_deps = [
     "//ios/chrome/browser/ui:ui_internal",
   ]
   libs = [ "UIKit.framework" ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "browser_view_wrangler_unittest.mm",
     "main_coordinator_unittest.mm",
     "main_view_controller_unittest.mm",
   ]
   deps = [
     ":main",
     "//base",
     "//components/bookmarks/test",
     "//ios/chrome/browser/bookmarks",
     "//ios/chrome/browser/browser_state:test_support",
     "//ios/chrome/browser/tabs",
     "//ios/chrome/browser/ui:ui_internal",
     "//ios/web:test_support",
     "//testing/gtest",
   ]
 }