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://golang.org/cl/95030044

argandas/tools,golang/tools,jdhenke/tools,glasser/golang-tools,dmitris/tools,magiccao/tools,cw2018/tools,josharian/tools,glasser/golang-tools,cmars/tools,springning/tools,runcom/tools,golang/tools,nicot/tools,golang/tools,syreclabs/go-tools,dmitris/tools,josharian/tools,cmars/tools,cw2018/tools,golang/tools,cw2018/tools,sshetty10/tools,springning/tools,josharian/tools,syreclabs/go-tools,golang/tools,glasser/golang-tools,glasser/golang-tools,4ad/tools,cw2018/tools,nicot/tools,argandas/tools,cmars/tools,4ad/tools,springning/tools,argandas/tools,syreclabs/go-tools,matryer/tools,maddyonline/tools,matryer/tools,josharian/tools,runcom/tools,dmitris/tools,4ad/tools,syreclabs/go-tools,nicot/tools,jdhenke/tools,maddyonline/tools,sshetty10/tools,maddyonline/tools,nicot/tools,sshetty10/tools,springning/tools,magiccao/tools,syreclabs/go-tools,sshetty10/tools,runcom/tools,magiccao/tools,argandas/tools,matryer/tools,jdhenke/tools,jdhenke/tools,runcom/tools,dmitris/tools,matryer/tools,maddyonline/tools,syreclabs/go-tools,magiccao/tools,cmars/tools