Automated rounding of content window dimensions

I've written a small patch for torbutton that forces the content ("gBrowser") to have dimensions be a multiple of 200x200. In other words, window.innerWidth and window.innerHeight, and similar calls, always return a rounded number.

This should at least provide some protection to users who resize or maximize their Tor Browser window with JS activated.

I haven't dealt with the zooming issue here, but that would be an interesting next step.