Provide the same scrollbar size across different platforms

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.

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