Apply referer policy to window.name

We should apply the user's chosen referer policy to window.name. In other words, if a url has a blank referer, we should clear out window.name for that window. With even the default referer policy, this will have the effect of wiping window.name between user-entered URLs.

This mechanism is also handy because it is currently impossible in XPCOM to distinguish user-entered URLs from javascript redirects of document.location.