IPC: Introduce MessageAttachment and PlatformFileAttachment.

ipc/ipc_message_attachment.h

Index: ipc/ipc_message_attachment.h
diff --git a/ipc/ipc_message_attachment.h b/ipc/ipc_message_attachment.h
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+// 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_MESSAGE_ATTACHMENT_H_
+#define IPC_IPC_MESSAGE_ATTACHMENT_H_
+
+#include "base/memory/ref_counted.h"
+#include "ipc/ipc_export.h"
+
+namespace IPC {
+
+// Auxiliary data sent with |Message|. This can be a platform file descriptor
+// or a mojo |MessagePipe|. |GetType()| returns the type of the subclass.
+class IPC_EXPORT MessageAttachment
+    : public base::RefCounted<MessageAttachment> {
+ public:
+  enum Type {
+    TYPE_PLATFORM_FILE,     // The instance is |PlatformFileAttachment|.
+    TYPE_MOJO_MESSAGE_PIPE, // The instance is a mojo-based class.
+  };
+
+  virtual Type GetType() const = 0;
+
+ protected:
+  friend class base::RefCounted<MessageAttachment>;
+  virtual ~MessageAttachment();
+};
+
+}  // namespace IPC
+
+#endif  // IPC_IPC_MESSAGE_ATTACHMENT_H_