Things unique to Chrome that don't come from the OS: V8 Javascript engine, process isolation model, networking including new protocols like SPDY and possibly QUIC [1], auto-update (for browser and all apps and extensions) Chrome sync (for bookmarks, open tabs, apps, extensions). Don't forget that the Chrome team is now a major contributor to WebKit.

[1] https://plus.google.com/u/0/100132233764003563318/posts/b36w...