This is needed so another CL in the main repo can keep the pointer
around to pass through Transport.CloseIdleConnections from the http1
transport.

This CL doesn't modify the exported ConfigureTransport
signature. We'll use the private one in the standard library for
now. (since it gets bundled into the same package)

Updates golang/go#13975

Change-Id: I824e9ac4a44616c8c2a480f83bd3dc62bffc30e4
Reviewed-on: https://go-review.googlesource.com/18678Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>