This isn't actually used yet, but implements CTR-DRBG from SP 800-90Ar1.
Specifically, it always uses AES-256 and no derivation function.
Change-Id: Ie82b829590226addd7c165eac410a5d584858bfd
Reviewed-on: https://boringssl-review.googlesource.com/14891
Reviewed-by: Adam Langley <agl@google.com>
