cmd/tip: enable playground on tip.golang.org
cmd/tip: enable playground on tip.golang.org

Updates golang/go#28536

Change-Id: I3ece72aacd4635f4580be404672394a47791a1e8
Reviewed-on: https://go-review.googlesource.com/c/146917
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools