Add XChaCha20-Poly1305 AEAD.

This is a version of ChaCha20-Poly1305 that takes a 24-byte nonce,
making the nonce suitable for random generation. It's compatible with
the AEAD of the same name in libsodium.

Change-Id: Ie8b20ba551e5a290b390d362e487f06377166f4c
Reviewed-on: https://boringssl-review.googlesource.com/30384
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: David Benjamin <davidben@google.com>

sources.cmake

diff --git a/sources.cmake b/sources.cmake
index f0c953b..94edcb3 100644
--- a/sources.cmake
+++ b/sources.cmake
@@ -22,6 +22,7 @@
   crypto/cipher_extra/test/aes_256_gcm_siv_tests.txt
   crypto/cipher_extra/test/aes_256_gcm_tests.txt
   crypto/cipher_extra/test/chacha20_poly1305_tests.txt
+  crypto/cipher_extra/test/xchacha20_poly1305_tests.txt
   crypto/cipher_extra/test/cipher_tests.txt
   crypto/cipher_extra/test/des_ede3_cbc_sha1_tls_implicit_iv_tests.txt
   crypto/cipher_extra/test/des_ede3_cbc_sha1_tls_tests.txt