go.tools/playground/socket: require origin to set up socket handler
go.tools/playground/socket: require origin to set up socket handler

This prevents cross-site request forgery attacks.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/95030044

t-yuki/go.tools.old,t-yuki/go.tools.old,t-yuki/go.tools.old