IPC::Message Refactoring: Move POSIX specific bits to PlatformFileAttachment

ipc/ipc_platform_file_attachment.h

-// Copyright (c) 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_IPC_PLATFORM_FILE_ATTACHMENT_H_
-#define IPC_IPC_PLATFORM_FILE_ATTACHMENT_H_
-
-#include "base/files/file.h"
-#include "ipc/ipc_message_attachment.h"
-
-namespace IPC {
-namespace internal {
-
-// A platform file that is sent over |Channel| as a part of |Message|.
-// |PlatformFileAttachment| doesn't own |file_|. The lifecycle of |file_| is
-// managed by |MessageAttachmentSet|.
-class PlatformFileAttachment : public MessageAttachment {
- public:
-  explicit PlatformFileAttachment(base::PlatformFile file);
-
-  Type GetType() const override;
-  base::PlatformFile file() const { return file_; }
-
- private:
-  ~PlatformFileAttachment() override;
-
-  base::PlatformFile file_;
-};
-
-base::PlatformFile GetPlatformFile(scoped_refptr<MessageAttachment> attachment);
-
-}  // namespace internal
-}  // namespace IPC
-
-#endif  // IPC_IPC_PLATFORM_FILE_ATTACHMENT_H_