[ObjC ARC] Converts ios/chrome/browser/ui/collection_view/cells:test_support to ARC.

ios/chrome/browser/ui/collection_view/cells/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("cells") {
   sources = [
     "MDCCollectionViewCell+Chrome.h",
     "MDCCollectionViewCell+Chrome.mm",
     "activity_indicator_cell.h",
     "activity_indicator_cell.mm",
@@ skipping 23 matching lines @@
     "//url",
   ]
   public_deps = [
     "//ios/third_party/material_components_ios",
   ]
 
   configs += [ "//build/config/compiler:enable_arc" ]
 }
 
 source_set("test_support") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "test_utils.h",
     "test_utils.mm",
   ]
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
     "MDCCollectionViewCell+Chrome_unittest.mm",
     "activity_indicator_cell_unittest.mm",
     "collection_view_account_item_unittest.mm",
     "collection_view_detail_item_unittest.mm",
     "collection_view_footer_item_unittest.mm",
     "collection_view_item_unittest.mm",
     "collection_view_switch_item_unittest.mm",
     "collection_view_text_item_unittest.mm",
   ]
 
   deps = [
     ":cells",
     ":test_support",
     "//ios/third_party/material_components_ios",
     "//testing/gtest",
   ]
 
   configs += [ "//build/config/compiler:enable_arc" ]
 }