This is used for fallback from DTLS to SDES encryption, which we probably still
want to support. Setting a certificate puts the DTLS transport in a "DTLS
enabled" mode, so it should be delayed until SDP with "a=fingerprint" is set.
BUG=webrtc:6972
Review-Url: https://codereview.webrtc.org/2641633002
Cr-Commit-Position: refs/heads/master@{#16199}