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

ios/chrome/browser/ui/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.
 
 bundle_data("resources") {
   sources = [
     "resources/tabstrip_background_tab@2x~ipad.png",
     "resources/tabstrip_background_tab~ipad.png",
     "resources/tabstrip_foreground_tab@2x~ipad.png",
     "resources/tabstrip_foreground_tab~ipad.png",
@@ skipping 58 matching lines @@
     "//ios/third_party/material_components_ios",
     "//ios/web",
     "//third_party/google_toolbox_for_mac",
     "//ui/base",
     "//ui/gfx",
   ]
   libs = [ "UIKit.framework" ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "tab_strip_controller_unittest.mm",
   ]
   deps = [
     ":tabs",
     "//base",
     "//ios/chrome/browser/browser_state:test_support",
     "//ios/chrome/browser/sessions:test_support",
     "//ios/chrome/browser/tabs",
@@ skipping 15 matching lines @@
     "//ios/chrome/app/strings",
     "//ios/chrome/browser",
     "//ios/chrome/browser/ui",
     "//ios/chrome/test/app:test_support",
     "//ios/chrome/test/earl_grey:test_support",
     "//ios/third_party/earl_grey",
     "//ui/base",
   ]
   libs = [ "XCTest.framework" ]
 }