SSLECDHContext has the acronyms problem, so I went with SSLKeyShare to
match the TLS 1.3 terminology. It's also a little shorter. Accept and
Finish, for now, take raw output pointers in anticipation of some
bssl::Array and maybe bssl::CleansedArray types.
Bug: 132
Change-Id: I427c7c0eac95704f3ad093676c504c2848f5acb9
Reviewed-on: https://boringssl-review.googlesource.com/18265
Reviewed-by: Steven Valdez <svaldez@google.com>
