Add missing IWYU message_loop.h includes.

net/disk_cache/simple/simple_index_file_unittest.cc

 // Copyright (c) 2011 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 "net/disk_cache/simple/simple_index_file.h"
 
 #include <memory>
 
 #include "base/files/file.h"
 #include "base/files/file_util.h"
 #include "base/files/scoped_temp_dir.h"
 #include "base/hash.h"
 #include "base/location.h"
 #include "base/logging.h"
+#include "base/message_loop/message_loop.h"
 #include "base/pickle.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/threading/thread.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/time/time.h"
 #include "net/base/cache_type.h"
 #include "net/base/test_completion_callback.h"
 #include "net/disk_cache/disk_cache_test_util.h"
 #include "net/disk_cache/simple/simple_backend_impl.h"
@@ skipping 372 matching lines @@
   closure.WaitForResult();
 
   // Check that the temporary file was deleted and the index file was created.
   EXPECT_FALSE(base::PathExists(simple_index_file.GetTempIndexFilePath()));
   EXPECT_TRUE(base::PathExists(simple_index_file.GetIndexFilePath()));
 }
 
 #endif  // defined(OS_POSIX)
 
 }  // namespace disk_cache