[ObjC ARC] Converts ios/chrome/test/base:unit_tests to ARC.

ios/chrome/test/base/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("base") {
   testonly = true
   sources = [
     "scoped_block_swizzler.h",
     "scoped_block_swizzler.mm",
   ]
   deps = [
     "//base",
   ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "scoped_block_swizzler_unittest.mm",
   ]
   deps = [
     ":base",
     "//base",
     "//testing/gtest",
   ]
 }
 
 source_set("perf_test_support") {
   testonly = true
   sources = [
     "perf_test_ios.h",
     "perf_test_ios.mm",
   ]
   deps = [
     "//base",
     "//ios/chrome/browser/web:web_internal",
     "//ios/chrome/test:test_support",
     "//ios/web:test_support",
   ]
 }