According to iFixIts teardown: https://www.ifixit.com/Teardown/Chromecast+2015+Teardown/501...
It has a Marvell Avastar 88W8887 chip, which has NFC built in. http://www.marvell.com/wireless/88W8887/ as well as FM Radio.
I said this should be used for "Tap and Play"