net/http: make Transport.CloseIdleConnections close non-bundled http2.Transport

net/http: make Transport.CloseIdleConnections close non-bundled http2.Transport

Previously Transport.CloseIdleConnections only closed the HTTP/2
Transport's idle connections if the HTTP/2 transport was configured
automatically via the bundled copy (in h2_bundle.go).

This makes it also work if the user called http2.ConfigureTransport
themselves using golang.org/x/net/http2 instead of the bundled copy.

No tests because we have no current way to run such cross-repo tests,
at least in any efficient or non-flaky way.

Tested by hand that:

    package main

    import (
        "net/http"

        "golang.org/x/net/http2"
    )

    func main() {
        tr := &http.Transport{}
        http2.ConfigureTransport(tr)
        tr.CloseIdleConnections()
    }

... now works and calls the x/net/http2.Transport.CloseIdleConnections
code. (I threw in a print statement locally)

Fixes #22891 once CL 123656 is also in.

Change-Id: Id697fd3e7877c3a988bc3c3368b88940ba56cfd0
Reviewed-on: https://go-review.googlesource.com/123657
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>