Add missing IWYU message_loop.h includes.

ppapi/host/resource_message_filter_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 <stdint.h>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/location.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/synchronization/waitable_event.h"
 #include "base/threading/thread.h"
 #include "ipc/ipc_message.h"
 #include "ppapi/c/pp_errors.h"
 #include "ppapi/host/host_message_context.h"
 #include "ppapi/host/resource_host.h"
 #include "ppapi/host/resource_message_filter.h"
 #include "testing/gtest/include/gtest/gtest.h"
@@ skipping 195 matching lines @@
   // destroyed before the task is run.
   main_message_loop.task_runner()->PostTask(
       FROM_HERE, base::Bind(&ResourceMessageFilterTest::TestHandleMessageImpl,
                             base::Unretained(this)));
 
   base::RunLoop().RunUntilIdle();
 }
 
 }  // namespace proxy
 }  // namespace ppapi