Add missing IWYU message_loop.h includes.

gpu/command_buffer/client/cmd_buffer_helper_test.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.
 
 // Tests for the Command Buffer Helper.
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <memory>
 #include <vector>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "gpu/command_buffer/client/cmd_buffer_helper.h"
 #include "gpu/command_buffer/service/command_buffer_service.h"
 #include "gpu/command_buffer/service/command_executor.h"
 #include "gpu/command_buffer/service/mocks.h"
 #include "gpu/command_buffer/service/transfer_buffer_manager.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace gpu {
 
@@ skipping 736 matching lines @@
   helper_->OrderingBarrier();
   flush_count2 = command_buffer_->FlushCount();
   helper_->OrderingBarrier();
   flush_count3 = command_buffer_->FlushCount();
 
   EXPECT_EQ(flush_count2, flush_count1 + 1);
   EXPECT_EQ(flush_count3, flush_count2 + 1);
 }
 
 }  // namespace gpu