If built with the "autocert" build tag, use
golang.org/x/crypto/autocert and for automatic TLS certs.
This will be used for https://beta.golang.org/ running on GCE.
Change-Id: Id0e385796a25d663708ea9bb65c45cb1471dd526
Reviewed-on: https://go-review.googlesource.com/33751
Reviewed-by: Chris Broadfoot <cbro@golang.org>
Brad Fitzpatrick