[Greasemonkey] Convert getContents to use nsIURI

[Jeremy Dunck]

> On a fresh install of FF 1.5b2 with GM 0.6.2, Greasemonkey decided to
> throw some kind of error (I can't remember what it was, I'm sorry),
> whenever trying to read a file off local disk. Changing to this
> interface solved that little problem.
>
> I also note that nsIIOService's newChannelFromURI does proxy
> processing, which is useful if getContents is ever used to retrieve
> remote documents (not likely).

So.. patch?