buffet: Add support for DEVICE_DELETED XMPP notification

When DEVICE_DELETED notification is received over XMPP channel,
buffet will remove any cloud registration information (credentials,
robot account) and close server connections (XMPP channel, etc).

BUG=brillo:1215
TEST=`FEATURES=test emerge-link buffet`
     Test manually on the device.

Change-Id: I86e19659b9fbc06685bcabb6c659a633e797cae5
Reviewed-on: https://chromium-review.googlesource.com/281666
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Trybot-Ready: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/buffet/notification/notification_delegate.h b/buffet/notification/notification_delegate.h
index b2d7184..529e39d 100644
--- a/buffet/notification/notification_delegate.h
+++ b/buffet/notification/notification_delegate.h
@@ -19,6 +19,8 @@
   virtual void OnPermanentFailure() = 0;
   // Called when a new command is sent via the notification channel.
   virtual void OnCommandCreated(const base::DictionaryValue& command) = 0;
+  // Called when DEVICE_DELETED notification is received.
+  virtual void OnDeviceDeleted(const std::string& device_id) = 0;
 
  protected:
   virtual ~NotificationDelegate() = default;