nit: Rename RtpDemuxer::sink_ to RtpDemuxer::ssrc_sinks_

Rationale:
1. sinks_ is not properly differentiated from rsid_sinks_.
2. Consistency with RtcpDemuxer.

BUG=webrtc:7135

Review-Url: https://codereview.webrtc.org/2958283002
Cr-Original-Commit-Position: refs/heads/master@{#18813}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: c3e3e60f592865b671175941a972dbf6961e1b38
2 files changed