Add missing IWYU message_loop.h includes.

components/password_manager/core/browser/password_store_unittest.cc

 // Copyright (c) 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.
 
 // The passwords in the tests below are all empty because PasswordStoreDefault
 // does not store the actual passwords on OS X (they are stored in the Keychain
 // instead). We could special-case it, but it is easier to just have empty
 // passwords. This will not be needed anymore if crbug.com/466638 is fixed.
 
 #include <stddef.h>
 
 #include <utility>
 
 #include "base/bind.h"
 #include "base/files/scoped_temp_dir.h"
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/strings/string_util.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/synchronization/waitable_event.h"
 #include "base/test/scoped_task_environment.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/time/time.h"
 #include "build/build_config.h"
 #include "components/password_manager/core/browser/affiliated_match_helper.h"
 #include "components/password_manager/core/browser/affiliation_service.h"
@@ skipping 875 matching lines @@
 
   store->ShutdownOnUIThread();
   base::RunLoop().RunUntilIdle();
 #if defined(OS_MACOSX)
   OSCryptMocker::TearDown();
 #endif
 }
 #endif
 
 }  // namespace password_manager