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

ios/chrome/browser/crash_report/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("crash_report") {
   sources = [
     "breakpad_helper.h",
     "breakpad_helper.mm",
     "crash_keys.cc",
     "crash_keys.h",
@@ skipping 38 matching lines @@
     "//ios/chrome/browser/browser_state",
     "//ios/chrome/browser/sessions",
     "//ios/chrome/browser/tabs",
     "//ios/web",
     "//net",
     "//ui/base",
   ]
 }
 
 source_set("unit_tests") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "breakpad_helper_unittest.mm",
     "crash_restore_helper_unittest.mm",
   ]
   deps = [
     ":crash_report",
     ":crash_report_internal",
     "//base",
     "//breakpad:client",
     "//ios/chrome/browser/browser_state",
     "//ios/chrome/browser/browser_state:test_support",
     "//ios/chrome/browser/sessions",
     "//ios/chrome/test/base",
     "//ios/chrome/test/ocmock",
     "//ios/web:test_support",
     "//testing/gmock",
     "//testing/gtest",
     "//third_party/ocmock",
   ]
 }