Delete class SSRCDatabase, and its global ssrc registry,
and the method RTPSender::GenerateNewSSRC.

It's now mandatory for higher layers to call SetSSRC, RTPSender
no longer allocates any ssrc by default.

BUG=webrtc:4306,webrtc:6887

Review-Url: https://codereview.webrtc.org/2644303002
Cr-Original-Commit-Position: refs/heads/master@{#16670}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: b78d4d13835f628e722a57abae2bf06ba3655921
13 files changed