ipc: Make a new class PlaceholderBrokerableAttachment.

ipc/placeholder_brokerable_attachment.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_PLACEHOLDER_BROKERABLE_ATTACHMENT_H_
+#define IPC_PLACEHOLDER_BROKERABLE_ATTACHMENT_H_
+
+#include "ipc/brokerable_attachment.h"
+#include "ipc/ipc_export.h"
+
+namespace IPC {
+
+// This subclass of BrokerableAttachment has an AttachmentId, and nothing else.
+// It is intended to be replaced by the attachment broker.
+class IPC_EXPORT PlaceholderBrokerableAttachment : public BrokerableAttachment {
+ public:
+  PlaceholderBrokerableAttachment(const AttachmentId& id)
+      : BrokerableAttachment(id){};
+  BrokerableType GetBrokerableType() const override;
+
+ protected:
+  ~PlaceholderBrokerableAttachment() override{};
+  DISALLOW_COPY_AND_ASSIGN(PlaceholderBrokerableAttachment);
+};
+
+}  // namespace IPC
+
+#endif  // IPC_PLACEHOLDER_BROKERABLE_ATTACHMENT_H_