/usr/lib/mono/4.5/Mono.WebBrowser.dll is in libmono-webbrowser4.0-cil 4.2.1.102+dfsg2-7ubuntu4.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/mono/4.5/../gac/Mono.WebBrowser/4.0.0.0__0738eb9f132ed756/Mono.WebBrowser.dll