Use "UDP/TLS/RTP/SAVPF" profile in offer when DTLS-SRTP is used.

Tested that this doesn't break compatibility with Firefox or older
versions of Chrome, no matter which side generates the initial offer.

BUG=webrtc:2796

Review URL: https://codereview.webrtc.org/1219333002

Cr-Commit-Position: refs/heads/master@{#9589}
2 files changed