Fill in alg_bits for CHACHA20_POLY1305 ciphers.

Apparently we left those as zero. Oh well. This only affects
SSL_CIPHER_get_bits, but so long as we have the field, it ought to be correct.

Change-Id: I2878ec22c2f5a6263f805e04d9fd8448994639b7
Reviewed-on: https://boringssl-review.googlesource.com/5752
Reviewed-by: Adam Langley <agl@google.com>