Updates bundled http2 to x/net git rev a333c53 for:

   http2: add Transport support for IdleConnTimeout
   https://golang.org/cl/30075

And add tests.

The bundled http2 also includes a change adding a Ping method to
http2.ClientConn, but that type isn't exposed in the standard
library. Nevertheless, the code gets compiled and adds a dependency on
"crypto/rand", requiring an update to go/build's dependency
test. Because net/http already depends on crypto/tls, which uses
crypto/rand, it's not really a new dependency.

Fixes #16808

Change-Id: I1ec8666ea74762f27c70a6f30a366a6647f923f7
Reviewed-on: https://go-review.googlesource.com/30078
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>