This does not include an upgrade of golang.org/x/net.
This is optional and best done as a separate CL.

Change-Id: Ifecc3fb6e3b7fe026b4ddefbe637186a3445b0bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/204658
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>