ipc: Make MessageAttachment inherit from RefCountedThreadSafe.

ipc/mach_port_attachment_mac.h

 // Copyright 2015 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_MACH_PORT_ATTACHMENT_MAC_H_
 #define IPC_MACH_PORT_ATTACHMENT_MAC_H_
 
 #include <mach/mach.h>
 #include <stdint.h>
 
@@ skipping 49 matching lines @@
   // Returns the wire format of this attachment.
   WireFormat GetWireFormat(const base::ProcessId& destination) const;
 
   mach_port_t get_mach_port() const { return mach_port_; }
 
   // The caller of this method has taken ownership of |mach_port_|.
   void reset_mach_port_ownership() { owns_mach_port_ = false; }
 
  private:
   ~MachPortAttachmentMac() override;
-  mach_port_t mach_port_;
+  const mach_port_t mach_port_;
 
   // In the sender process, the attachment owns the Mach port of a newly created
   // message. The attachment broker will eventually take ownership, and set
   // this member to |false|.
   // In the destination process, the attachment never owns the Mach port. The
   // client code that receives the Chrome IPC message is always expected to take
   // ownership.
   bool owns_mach_port_;
   DISALLOW_COPY_AND_ASSIGN(MachPortAttachmentMac);
 };
 
 }  // namespace internal
 }  // namespace IPC
 
 #endif  // IPC_MACH_PORT_ATTACHMENT_MAC_H_