Add missing IWYU message_loop.h includes.

services/service_manager/background/tests/background_service_manager_unittest.cc

 // 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.
 
 #include "services/service_manager/background/background_service_manager.h"
 
 #include <memory>
 
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/values.h"
 #include "services/service_manager/background/tests/test.mojom.h"
 #include "services/service_manager/public/cpp/connector.h"
 #include "services/service_manager/public/cpp/service.h"
 #include "services/service_manager/public/cpp/service_context.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace service_manager {
 namespace {
@@ skipping 76 matching lines @@
   // Ask the service to quit itself.
   test_service->Quit();
   run_loop.Run();
 
   // The run loop was quit by the callback and not by something else.
   EXPECT_TRUE(callback_called);
 }
 
 }  // namespace
 }  // namespace service_manager