core/extension: Force relogin when getAuthToken returns undefined.
core/extension: Force relogin when getAuthToken returns undefined.

When a user has changed their password, chrome.getAuthToken returns
undefined. We cannot purge 'undefined' from the cache so we force
the complete logout of the stale account and try to login again.

Change-Id: Id277cb617c5159d17093524725ce0d8e2e5265c2

vanadium/js,vanadium/js,vanadium/js,vanadium/js