This change updates the vendored chacha20poly1305 package to match
revision 14f9af67c679edd414f72f13d67c917447113df2 of x/crypto.

Change-Id: I05a4ba86578b0f0cdb1ed7dd50fee3b38bb48cf5
Reviewed-on: https://go-review.googlesource.com/31312
Run-TryBot: Adam Langley <agl@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>