Upstream settled in this API, and it's also the one that we expect
internally and that third_party code will expect.

Change-Id: Id7af68cf0af1f2e4d9defd37bda2218d70e2aa7b
Reviewed-on: https://boringssl-review.googlesource.com/3542
Reviewed-by: Adam Langley <agl@google.com>