Browser only offers to save password, it doesn't do automatically, its user who does. Saving password is a privilege not a threat. If user are aware of consequences that saving password might bring, and still they do and something goes wrong unfortunately, then its user who is to blame not browser or a programmer.
Security is a big term the company use to sell their browser, so they actually don't allow much control over their software via javascript.
You may find some dirty hacks around, but that doesn't guarantee that it will work on all browsers or in future versions of the browsers.