Make attachment broker a subclass of IPC::Listener.

ipc/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_BROKERABLE_ATTACHMENT_H_
 #define IPC_BROKERABLE_ATTACHMENT_H_
 
 #include <stdint.h>
 
 #include "base/macros.h"
@@ skipping 19 matching lines @@
   // The identifier is unique across all Chrome processes.
   AttachmentId GetIdentifier() const;
 
   // Returns TYPE_BROKERABLE_ATTACHMENT
   Type GetType() const override;
 
   virtual BrokerableType GetBrokerableType() const = 0;
 
  protected:
   BrokerableAttachment();
+  explicit BrokerableAttachment(const AttachmentId& id);
   ~BrokerableAttachment() override;
 
  private:
   // This member uniquely identifies a BrokerableAttachment across all Chrome
   // processes.
   const AttachmentId id_;
   DISALLOW_COPY_AND_ASSIGN(BrokerableAttachment);
 };
 
 }  // namespace IPC
 
 #endif  // IPC_BROKERABLE_ATTACHMENT_H_