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

ios/chrome/browser/ui/collection_view/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("collection_view") {
   sources = [
     "collection_view_controller.h",
     "collection_view_controller.mm",
     "collection_view_model.h",
     "collection_view_model.mm",
@@ skipping 19 matching lines @@
     "//base",
     "//ios/chrome/browser/ui/collection_view/cells",
     "//ios/chrome/test:test_support",
     "//ios/third_party/material_components_ios",
     "//testing/gtest",
     "//ui/base",
   ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "collection_view_controller_unittest.mm",
     "collection_view_model_unittest.mm",
   ]
   deps = [
     ":collection_view",
     "//base",
     "//ios/chrome/browser/ui/collection_view/cells",
     "//ios/chrome/test:test_support",
     "//ios/chrome/test/base",
     "//ios/third_party/material_components_ios",
     "//testing/gtest",
   ]
 }