Delete class SSRCDatabase, and its global ssrc registry.

webrtc/logging/rtc_event_log/rtc_event_log.proto

Index: webrtc/logging/rtc_event_log/rtc_event_log.proto
diff --git a/webrtc/logging/rtc_event_log/rtc_event_log.proto b/webrtc/logging/rtc_event_log/rtc_event_log.proto
index 0da910a29f51ab62e1e84440a520260bdc2f1f2f..b1ece47b6d342e65514e4edb1f497fa75529f95e 100644
--- a/webrtc/logging/rtc_event_log/rtc_event_log.proto
+++ b/webrtc/logging/rtc_event_log/rtc_event_log.proto
@@ -102,6 +102,8 @@ message RtcpPacket {
 }
 
 message AudioPlayoutEvent {
+  // TODO(ivoc): Rename, we currently use the "remote" ssrc, i.e. identifying
+  // the receive stream, while local_ssrc identifies the send stream, if any.
   // required - The SSRC of the audio stream associated with the playout event.
   optional uint32 local_ssrc = 2;
 }