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

ios/chrome/browser/ui/bookmarks/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("bookmarks") {
   configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "bookmark_collection_cells.h",
     "bookmark_collection_cells.mm",
     "bookmark_collection_view.h",
@@ skipping 124 matching lines @@
     "//components/bookmarks/test",
     "//ios/chrome/browser",
     "//ios/chrome/browser/bookmarks",
     "//ios/chrome/browser/browser_state:test_support",
     "//ios/web:test_support",
     "//testing/gtest",
   ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "bookmark_home_view_controller_unittest.mm",
     "bookmark_ios_unittest.mm",
     "bookmark_position_cache_unittest.mm",
   ]
   public_deps = [
     ":unit_tests_arc",
   ]
   deps = [
@@ skipping 107 matching lines @@
     "resources/bookmark_white_edit@2x.png",
     "resources/bookmark_white_edit@3x.png",
     "resources/bookmark_white_move.png",
     "resources/bookmark_white_move@2x.png",
     "resources/bookmark_white_move@3x.png",
   ]
   outputs = [
     "{{bundle_resources_dir}}/{{source_file_part}}",
   ]
 }