At some point we switched tip.golang.org to run in GKE, which
terminates TLS directly on port 443. This requires a new technique
for detecting a plain HTTP connection. In addition we may want to run
talks.golang.org on App Engine Flex, which uses an X-Forwarded-Proto
header to indicate HTTP, so let's prepare for that possibility.
Fixesgolang/go#19759.
Change-Id: Iddc567214c5d28f61c405db065aa1b3f2c92fd85
Reviewed-on: https://go-review.googlesource.com/38800
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
