$ sudo apt-get update
$ sudo apt-get install lightdm-webkit-greeter 

lightdm-webkit-greeter is installed in your system.

Make ensure the lightdm-webkit-greeter package were installed using the commands given below,

$ sudo dpkg-query -l | grep lightdm-webkit-greeter *

You will get with lightdm-webkit-greeter package name, version, architecture and description in a table

lightdm-webkit-greeter package basic information: