http://samy.pl/pwnat/ allows to establish a bidirectional connection between two machines behind NATs, but one of them must know the IP of the other's NAT, so this still requires a peer discovery system.
Also, I don't know if Chrome's socket API is sufficiently low level to craft ICMP packets. If I understand properly, it only offers a TCP socket.
As a matter of fact you don't need socket API or webRTC or whatsoever, Chrome has established an XMPP and jingle p2p connection for sync bookmarks, etc. It's not open API for Javascript yet, but I defintely see the potential.
Also, I don't know if Chrome's socket API is sufficiently low level to craft ICMP packets. If I understand properly, it only offers a TCP socket.