Add missing IWYU message_loop.h includes.

base/trace_event/trace_event_system_stats_monitor_unittest.cc

 // Copyright 2013 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 "base/trace_event/trace_event_system_stats_monitor.h"
 
 #include <sstream>
 #include <string>
 
 #include "base/macros.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/trace_event/trace_event_impl.h"
 #include "build/build_config.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace base {
 namespace trace_event {
 
 #if !defined(OS_IOS)
 // Tests for the system stats monitor.
@@ skipping 37 matching lines @@
   EXPECT_FALSE(system_stats_monitor->IsTimerRunningForTest());
 
   // Deleting the observer removes it from the TraceLog observer list.
   system_stats_monitor.reset();
   EXPECT_EQ(0u, TraceLog::GetInstance()->GetObserverCountForTest());
 }
 #endif  // !defined(OS_IOS)
 
 }  // namespace trace_event
 }  // namespace base