Add missing IWYU message_loop.h includes.

chrome/browser/extensions/app_data_migrator_unittest.cc

 // Copyright 2015 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 <stdint.h>
 
 #include <string>
 #include <utility>
 
 #include "base/callback_forward.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/threading/sequenced_worker_pool.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "chrome/browser/extensions/app_data_migrator.h"
 #include "chrome/browser/extensions/extension_special_storage_policy.h"
 #include "chrome/test/base/testing_profile.h"
 #include "content/public/browser/browser_thread.h"
 #include "content/public/browser/indexed_db_context.h"
 #include "content/public/browser/storage_partition.h"
 #include "content/public/test/test_browser_thread_bundle.h"
@@ skipping 256 matching lines @@
   content::StoragePartition* new_partition =
       content::BrowserContext::GetStoragePartitionForSite(profile_.get(),
                                                           extension_url);
 
   ASSERT_NE(new_partition->GetPath(), default_partition_->GetPath());
 
   VerifyTestFilesMigrated(new_partition, new_ext.get());
 }
 
 }  // namespace extensions