This avoids callers having to worry about |CRYPTO_add| and what the
correct lock to use it with is. (Esp since we'll probably change the way
that reference counts work in the future.)
Change-Id: I972bf0cc3be6099e0255e64a0fd50249062d1eb4
Reviewed-on: https://boringssl-review.googlesource.com/4623
Reviewed-by: Adam Langley <agl@google.com>