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

ios/chrome/browser/ui/ntp/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/bookmarks_bar_bg.png",
     "resources/bookmarks_bar_bg@2x.png",
     "resources/bookmarks_bar_bg@2x~ipad.png",
     "resources/bookmarks_bar_bg_pressed@2x~ipad.png",
@@ skipping 188 matching lines @@
     "//url",
   ]
   libs = [
     "CoreGraphics.framework",
     "QuartzCore.framework",
     "UIKit.framework",
   ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "centering_scrollview_unittest.mm",
     "google_landing_controller_unittest.mm",
     "most_visited_cell_unittest.mm",
     "new_tab_page_bar_unittest.mm",
     "new_tab_page_controller_unittest.mm",
     "notification_promo_whats_new_unittest.mm",
   ]
   deps = [
@@ skipping 54 matching lines @@
     "new_tab_page_perftest.mm",
   ]
   deps = [
     "//base/test:test_support",
     "//ios/chrome/browser/sessions",
     "//ios/chrome/browser/tabs",
     "//ios/chrome/browser/test:perf_test_support",
     "//ios/chrome/browser/ui:ui_internal",
   ]
 }