This change was originally made in https://golang.org/cl/46631, however,
that change was applited to net/http/h2_bundle.go instead of x/net/http2.

Updates golang/go#20784

Change-Id: I947fa4c19f3efc400856573768140bece28276a2
Reviewed-on: https://go-review.googlesource.com/80135
Run-TryBot: Tom Bergan <tombergan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>