ipc: Move methods into AttachmentBroker interface.

ipc/attachment_broker_privileged.h

Index: ipc/attachment_broker_privileged.h
diff --git a/ipc/attachment_broker_privileged.h b/ipc/attachment_broker_privileged.h
index 0d5f4a871f74187ce60e710b0aace72cf894e332..387dbe5c9f77b714ccf08f511fd74c7a1c1c2c95 100644
--- a/ipc/attachment_broker_privileged.h
+++ b/ipc/attachment_broker_privileged.h
@@ -33,12 +33,9 @@ class IPC_EXPORT AttachmentBrokerPrivileged : public IPC::AttachmentBroker {
    // global attachment broker.
   static scoped_ptr<AttachmentBrokerPrivileged> CreateBroker();
 
-  // Each unprivileged process should have one IPC channel on which it
-  // communicates attachment information with the broker process. In the broker
-  // process, these channels must be registered and deregistered with the
-  // Attachment Broker as they are created and destroyed.
-  void RegisterCommunicationChannel(Endpoint* endpoint);
-  void DeregisterCommunicationChannel(Endpoint* endpoint);
+  // AttachmentBroker overrides.
+  void RegisterCommunicationChannel(Endpoint* endpoint) override;
+  void DeregisterCommunicationChannel(Endpoint* endpoint) override;
 
  protected:
   // Returns the sender whose peer's process id is |id|.