Add missing IWYU message_loop.h includes.

chrome/browser/extensions/activity_log/counting_policy_unittest.cc

 // Copyright 2013 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.
 
 #include "chrome/browser/extensions/activity_log/counting_policy.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <memory>
 #include <utility>
 
 #include "base/cancelable_callback.h"
 #include "base/command_line.h"
 #include "base/location.h"
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/strings/string_split.h"
 #include "base/strings/stringprintf.h"
 #include "base/synchronization/waitable_event.h"
 #include "base/test/simple_test_clock.h"
 #include "base/test/test_timeouts.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/values.h"
 #include "build/build_config.h"
@@ skipping 1253 matching lines @@
 
   action_ids.push_back(2);
   CheckRemoveActions(
       policy, action_ids, base::Bind(&CountingPolicyTest::Action2Deleted));
   action_ids.clear();
 
   policy->Close();
 }
 
 }  // namespace extensions