Add missing IWYU message_loop.h includes.

chrome/browser/sessions/session_service_unittest.cc

 // Copyright 2012 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/sessions/session_service.h"
 
 #include <stddef.h>
 
 #include <memory>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/files/file_util.h"
 #include "base/files/scoped_temp_dir.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/stl_util.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/synchronization/waitable_event.h"
 #include "base/time/time.h"
 #include "build/build_config.h"
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/chrome_notification_types.h"
@@ skipping 1019 matching lines @@
                             &cancelable_task_tracker);
   helper_.RunTaskOnBackendThread(
       FROM_HERE,
       base::Bind(&PostBackToThread,
                  base::Unretained(base::MessageLoop::current()),
                  base::Unretained(&run_loop)));
   delete helper_.ReleaseService();
   event.Signal();
   run_loop.Run();
 }