Provide the same scrollbar size across different platforms + versions (and subpixel entropy)

Scrollbar sizes are different on different platforms. But it seems that there are ways to split the Linux users into different buckets based on that: On a Debian Stretch system with XFCE I get 15px thickness on an Ubuntu 14.04 system with GNOME I get 13px thickness.

A test can be found on http://www.hackerfactor.com/blog/index.php?/archives/761-Exploiting-the-TOR-Browser.html.

One option mentioned in that blog post would be to provide 17px on all platforms.