Explore zoom-based alternatives to fixed window sizes

Right now, we set the size of new Tor Browser windows such that their content area is a 200x100 multiple. We also lie to content that the entire desktop resolution is this size.

However, this potentially leaks information for users who maximize their browser windows, as such windows will no longer be rounded.

We could play with zooming such that maximized windows do not reveal Firefox decoration sizes. We could also set the zoom level automatically such that we end up with a content window size of a 200x100 multiple as well.

Not sure how complicated this will be. See also legacy/trac#7255 (moved) for a potentially simpler stopgap.