Updates x/net/http2 to git rev 72aa00c6 for https://golang.org/cl/18721
(but actually at https://golang.org/cl/18722 now)

Fixes #14008

Change-Id: If05d5ad51ec0ba5ba7e4fe16605c0a83f0484bc8
Reviewed-on: https://go-review.googlesource.com/18723
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>