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-Commit-Position: refs/heads/master@{#18813}
diff --git a/webrtc/call/rtp_demuxer.cc b/webrtc/call/rtp_demuxer.cc
index e1957ed..b616f6f 100644
--- a/webrtc/call/rtp_demuxer.cc
+++ b/webrtc/call/rtp_demuxer.cc
@@ -27,7 +27,7 @@
 RtpDemuxer::RtpDemuxer() = default;
 
 RtpDemuxer::~RtpDemuxer() {
-  RTC_DCHECK(sinks_.empty());
+  RTC_DCHECK(ssrc_sinks_.empty());
   RTC_DCHECK(rsid_sinks_.empty());
 }
 
@@ -50,7 +50,7 @@
 
 bool RtpDemuxer::RemoveSink(const RtpPacketSinkInterface* sink) {
   RTC_DCHECK(sink);
-  return (RemoveFromMultimapByValue(&sinks_, sink) +
+  return (RemoveFromMultimapByValue(&ssrc_sinks_, sink) +
           RemoveFromMultimapByValue(&rsid_sinks_, sink)) > 0;
 }
 
@@ -59,14 +59,14 @@
   RTC_DCHECK(sink);
   // The association might already have been set by a different
   // configuration source.
-  if (!MultimapAssociationExists(sinks_, ssrc, sink)) {
-    sinks_.emplace(ssrc, sink);
+  if (!MultimapAssociationExists(ssrc_sinks_, ssrc, sink)) {
+    ssrc_sinks_.emplace(ssrc, sink);
   }
 }
 
 bool RtpDemuxer::OnRtpPacket(const RtpPacketReceived& packet) {
   ResolveAssociations(packet);
-  auto it_range = sinks_.equal_range(packet.Ssrc());
+  auto it_range = ssrc_sinks_.equal_range(packet.Ssrc());
   for (auto it = it_range.first; it != it_range.second; ++it) {
     it->second->OnRtpPacket(packet);
   }
diff --git a/webrtc/call/rtp_demuxer.h b/webrtc/call/rtp_demuxer.h
index 9ec9378..5060a80 100644
--- a/webrtc/call/rtp_demuxer.h
+++ b/webrtc/call/rtp_demuxer.h
@@ -71,12 +71,12 @@
 
   // This records the association SSRCs to sinks. Other associations, such
   // as by RSID, also end up here once the RSID, etc., is resolved to an SSRC.
-  std::multimap<uint32_t, RtpPacketSinkInterface*> sinks_;
+  std::multimap<uint32_t, RtpPacketSinkInterface*> ssrc_sinks_;
 
   // A sink may be associated with an RSID - RTP Stream ID. This tag has a
   // one-to-one association with an SSRC, but that SSRC is not yet known.
   // When it becomes known, the association of the sink to the RSID is deleted
-  // from this container, and moved into |sinks_|.
+  // from this container, and moved into |ssrc_sinks_|.
   std::multimap<std::string, RtpPacketSinkInterface*> rsid_sinks_;
 
   // Iterating over |rsid_sinks_| for each incoming and performing multiple