IPC: Add new classes required for attachment brokering.

ipc/brokerable_attachment.h

Index: ipc/brokerable_attachment.h
diff --git a/ipc/brokerable_attachment.h b/ipc/brokerable_attachment.h
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+// 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_BROKERABLE_ATTACHMENT_H_
+#define IPC_BROKERABLE_ATTACHMENT_H_
+
+#include <stdint.h>
+
+#include "base/macros.h"
+#include "ipc/ipc_export.h"
+#include "ipc/ipc_message_attachment.h"
+
+namespace IPC {
+
+// This subclass of MessageAttachment requires an AttachmentBroker to be
+// attached to a Chrome IPC message.
+class IPC_EXPORT BrokerableAttachment : public MessageAttachment {
+ public:
+  // An id uniquely identifies an attachment sent via a broker.
+  struct IPC_EXPORT AttachmentId {
+    uint32_t nonce[4];
+  };
+
+  // The identifier is unique across all Chrome processes.
+  AttachmentId GetIdentifier() const;
+
+ protected:
+  BrokerableAttachment();
+  ~BrokerableAttachment() override;
+
+ private:
+  // This member uniquely identifies a BrokerableAttachment across all Chrome
+  // processes.
+  AttachmentId id_;
+  DISALLOW_COPY_AND_ASSIGN(BrokerableAttachment);
+};
+
+}  // namespace IPC
+
+#endif  // IPC_BROKERABLE_ATTACHMENT_H_