Called when a client disconnects from |server|. |connection_id| uniquely identifies the connection. The client should release any data associated with |connection_id| when this function is called and |connection_id| should no longer be passed to cef_server_t functions. Disconnects can originate from either the client or the server. For example, the server will disconnect automatically after a cef_server_t::SendHttpXXXResponse function is called.