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

ios/chrome/browser/ui/omnibox/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("omnibox") {
   sources = [
     "omnibox_util.cc",
     "omnibox_util.h",
     "web_omnibox_edit_controller.cc",
     "web_omnibox_edit_controller.h",
@@ skipping 92 matching lines @@
   ]
   libs = [
     "CoreText.framework",
     "MobileCoreServices.framework",
     "QuartzCore.framework",
     "UIKit.framework",
   ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "omnibox_text_field_ios_unittest.mm",
   ]
   deps = [
     ":omnibox_internal",
     ":resources_unit_tests",
     "//base",
     "//ios/chrome/app/strings",
     "//ios/chrome/browser",
     "//testing/gtest",
     "//ui/base",
   ]
 }
 
 bundle_data("resources_unit_tests") {
   visibility = [ ":unit_tests" ]
   testonly = true
   sources = [
     "//ios/chrome/test/data/omnibox/selected_ranges.txt",
   ]
   outputs = [
     "{{bundle_resources_dir}}/" +
         "ios/chrome/test/data/omnibox/{{source_file_part}}",
   ]
 }