/usr/lib/mono/4.5/Microsoft.Web.Infrastructure.dll is in libmono-microsoft-web-infrastructure1.0-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/Microsoft.Web.Infrastructure/1.0.0.0__31bf3856ad364e35/Microsoft.Web.Infrastructure.dll