The server currently has no way to tell chrome that a given field should not be autofilled (i.e., to tell chrome not to trust its heuristics).
Chrome expects that the server returns a valid type or NO_SERVER_DATA. It has a DCHECK that the server does not return UNKNOWN_TYPE.
Chrome should be updated to accept UNKNOWN_TYPE as a valid response form the server. These predictions should be treated just like any other prediction from the server (as an override for the heuristics, but not the autocomplete attribute).
Comment 1 by bugdroid1@chromium.org
, Dec 8 2017