Cleanup remaining class/struct fwd declarations

mojo/edk/system/data_pipe_control_message.h

 // Copyright 2016 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.
 
 #ifndef MOJO_EDK_SYSTEM_DATA_PIPE_CONTROL_MESSAGE_H_
 #define MOJO_EDK_SYSTEM_DATA_PIPE_CONTROL_MESSAGE_H_
 
 #include <stdint.h>
 
 #include <memory>
 
 #include "mojo/edk/embedder/scoped_platform_handle.h"
 #include "mojo/edk/system/ports/port_ref.h"
 #include "mojo/public/c/system/macros.h"
 
 namespace mojo {
 namespace edk {
 
 class NodeController;
-class PortsMessage;
 
 enum DataPipeCommand : uint32_t {
   // Signal to the consumer that new data is available.
   DATA_WAS_WRITTEN,
 
   // Signal to the producer that data has been consumed.
   DATA_WAS_READ,
 };
 
 // Message header for messages sent over a data pipe control port.
 struct MOJO_ALIGNAS(8) DataPipeControlMessage {
   DataPipeCommand command;
   uint32_t num_bytes;
 };
 
 void SendDataPipeControlMessage(NodeController* node_controller,
                                 const ports::PortRef& port,
                                 DataPipeCommand command,
                                 uint32_t num_bytes);
 
 }  // namespace edk
 }  // namespace mojo
 
 #endif  // MOJO_EDK_SYSTEM_DATA_PIPE_CONTROL_MESSAGE_H_