IPC: Introduce MessageAttachment and 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_