/usr/lib/mono/4.5/System.Web.dll is in libmono-system-web4.0-cil 3.2.8+dfsg-4ubuntu1.1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/mono/4.5/../gac/System.Web/4.0.0.0__b03f5f7f11d50a3a/System.Web.dll