Add missing IWYU message_loop.h includes.

content/browser/appcache/appcache_storage_impl_unittest.cc

 // Copyright 2014 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 "content/browser/appcache/appcache_storage_impl.h"
 
 #include <stdint.h>
 
 #include <memory>
 #include <stack>
 #include <utility>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/callback.h"
 #include "base/files/file_util.h"
 #include "base/files/scoped_temp_dir.h"
 #include "base/location.h"
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/synchronization/waitable_event.h"
 #include "base/threading/thread.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "content/browser/appcache/appcache.h"
 #include "content/browser/appcache/appcache_backend_impl.h"
 #include "content/browser/appcache/appcache_database.h"
 #include "content/browser/appcache/appcache_entry.h"
 #include "content/browser/appcache/appcache_group.h"
 #include "content/browser/appcache/appcache_host.h"
@@ skipping 1995 matching lines @@
   RunTestOnIOThread(&AppCacheStorageImplTest::Reinitialize2);
 }
 
 TEST_F(AppCacheStorageImplTest, Reinitialize3) {
   RunTestOnIOThread(&AppCacheStorageImplTest::Reinitialize3);
 }
 
 // That's all folks!
 
 }  // namespace content