Add missing IWYU message_loop.h includes.

third_party/WebKit/Source/platform/scheduler/child/idle_helper_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 "platform/scheduler/child/idle_helper.h"
 
 #include <utility>
 
 #include "base/callback.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/test/simple_test_tick_clock.h"
 #include "cc/test/ordered_simple_task_runner.h"
 #include "platform/scheduler/base/real_time_domain.h"
 #include "platform/scheduler/base/task_queue.h"
 #include "platform/scheduler/base/task_queue_manager.h"
 #include "platform/scheduler/base/test_time_source.h"
 #include "platform/scheduler/child/scheduler_helper.h"
 #include "platform/scheduler/child/scheduler_tqm_delegate_for_test.h"
 #include "platform/scheduler/child/scheduler_tqm_delegate_impl.h"
@@ skipping 1086 matching lines @@
       expected_deadline);
   EXPECT_EQ(1u, idle_queue()->GetNumberOfPendingTasks());
   RunUntilIdle();
 
   EXPECT_EQ(1, run_count);
   EXPECT_EQ(expected_deadline, deadline_in_task);
 }
 
 }  // namespace scheduler
 }  // namespace blink