Port ssl3_{get,send}_server_key_exchange to EVP_Digest{Verify,Sign}*.

Minor change, but they're the users of the old API left within
BoringSSL.

Change-Id: Ic24e0d006c97fa5265abc3373d3f98aa8d2f8b1e
Reviewed-on: https://boringssl-review.googlesource.com/2100
Reviewed-by: Adam Langley <agl@google.com>
2 files changed