Updates x/net/http2 to git rev 313cf39 for CLs 23812 and 23880:

	http2: GotFirstResponseByte hook should only fire once
	http2: fix data race on pipe

Fixes #16000

Change-Id: I9c3f1b2528bbd99968aa5a0529ae9c5295979d1d
Reviewed-on: https://go-review.googlesource.com/23881Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>