We'll need this for handling name constraints during verification.

Change-Id: I4ef19d9489fb2a9ae9a62699d81cef92a21fda28
Reviewed-on: https://go-review.googlesource.com/62692
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>