Add missing IWYU message_loop.h includes.

components/sync/engine_impl/sync_scheduler_impl_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.
 
 #include "components/sync/engine_impl/sync_scheduler_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <vector>
 
 #include "base/bind.h"
 #include "base/location.h"
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/test/test_timeouts.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "components/sync/base/cancelation_signal.h"
 #include "components/sync/base/extensions_activity.h"
 #include "components/sync/base/model_type_test_util.h"
 #include "components/sync/engine_impl/backoff_delay_provider.h"
 #include "components/sync/engine_impl/cycle/test_util.h"
 #include "components/sync/syncable/test_user_share.h"
@@ skipping 1868 matching lines @@
   EXPECT_TRUE(GetBackedOffTypes().HasAll(themes_types));
   EXPECT_FALSE(GetBackedOffTypes().HasAll(typed_urls_types));
   EXPECT_TRUE(BlockTimerIsRunning());
   EXPECT_FALSE(scheduler()->IsBackingOff());
   EXPECT_FALSE(scheduler()->IsCurrentlyThrottled());
 
   StopSyncScheduler();
 }
 
 }  // namespace syncer