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

ios/chrome/browser/ui/stack_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.
 
 bundle_data("resources") {
   sources = [
     "resources/border_frame_bottom.png",
     "resources/border_frame_bottom@2x.png",
     "resources/border_frame_bottom@3x.png",
     "resources/border_frame_incognito_bottom.png",
@@ skipping 88 matching lines @@
     "//ui/gfx",
   ]
   libs = [
     "CoreGraphics.framework",
     "QuartzCore.framework",
     "UIKit.framework",
   ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "card_set_unittest.mm",
     "card_stack_layout_manager_unittest.mm",
     "stack_card_unittest.mm",
     "stack_view_controller_unittest.mm",
   ]
   deps = [
     ":stack_view",
     "//base",
@@ skipping 40 matching lines @@
     "//ios/chrome/browser/snapshots",
     "//ios/chrome/browser/tabs",
     "//ios/chrome/browser/test:perf_test_support",
     "//ios/chrome/browser/ui",
     "//ios/chrome/browser/ui:ui_internal",
     "//ios/web",
     "//net",
   ]
   libs = [ "UIKit.framework" ]
 }