Updates x/net/http2 to git rev 62685c2 for https://golang.org/cl/19483

Change-Id: Id01331cdba03934a6e55e55ad9c2ae27461ba149
Reviewed-on: https://go-review.googlesource.com/19484Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>