Support intermediate certificates for OpenSSL client auth.

Change the client auth callback to the 'cert_cb' callback which leaves the
callback in charge of installing the certificate, private key, etc. This lets
us attach a certificate chain.

BUG=393323

Review URL: https://codereview.chromium.org/441823002

git-svn-id: http://src.chromium.org/svn/trunk/src/net@289834 4ff67af0-8c30-449e-8e8b-ad334ec8d88c