vendor/golang_org/x/crypto/chacha20poly1305: revendor

Brings in chacha20poly1305 directory from golang.org/x/crypto revision
453249f01cfeb54c3d549ddb75ff152ca243f9d8, adding:

CL 35874: crypto/chacha20poly1305/internal/chacha20: add missing copyright header
CL 35875: crypto/chacha20poly1305: rename test vectors file

Fixes #19155.

Change-Id: I25cf83d060113f6b2a197f243a25614440008f7e
Reviewed-on: https://go-review.googlesource.com/37408Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/vendor/golang_org/x/crypto/chacha20poly1305/internal/chacha20/chacha_test.go

+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
 package chacha20
 
 import (