SRTP, in most cases, at least for point-to-point encryption.

I'm not too sure about end-to-end encryption in WebRTC, though; I remember reading that something different happens there [1].

[1] https://webrtchacks.com/you-dont-have-end-to-end-encryption-... and https://webrtchacks.com/true-end-to-end-encryption-with-webr...