The last bus to the Westin leaves at 21:40 from Terminal 1, where CX arrives.
http://www.limousinebus.co.jp/e/time...a_ebisu_h.html
If you miss this one, you can take another bus to Tokyo City Air Terminal or another hotel in central Tokyo (Shinjuku/Akasaka/Ginza/Kudan, but not Ikebukuro, a bit far). Then take a taxi from there. You can also take subway or train but if you are not familiar and with luggage, I would not recommend it.
You also have train options from Narita.
This last CX flight of the day from HKG can be a mess when it arrives late. Narita airport is so inconveniently located.