Cleanup remaining class/struct fwd declarations

ipc/ipc_message.h

 // 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.
 
 #ifndef IPC_IPC_MESSAGE_H_
 #define IPC_IPC_MESSAGE_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 12 matching lines @@
 
 namespace IPC {
 
 namespace internal {
 class ChannelReader;
 }  // namespace internal
 
 //------------------------------------------------------------------------------
 
 struct LogData;
-class MessageAttachment;
 class MessageAttachmentSet;
 
 class IPC_EXPORT Message : public base::Pickle {
  public:
   enum PriorityValue {
     PRIORITY_LOW = 1,
     PRIORITY_NORMAL,
     PRIORITY_HIGH
   };
 
@@ skipping 253 matching lines @@
   MSG_ROUTING_NONE = -2,
 
   // indicates a general message not sent to a particular tab.
   MSG_ROUTING_CONTROL = INT32_MAX,
 };
 
 #define IPC_REPLY_ID 0xFFFFFFF0  // Special message id for replies
 #define IPC_LOGGING_ID 0xFFFFFFF1  // Special message id for logging
 
 #endif  // IPC_IPC_MESSAGE_H_