Add ctor to RtpDemuxerCriteria to allow supplying 'mid' and 'rsid'.

Since another CL [1] grew a bit in scope, this CL needs to land first,
coupled with downstream updates.

[1]: https://webrtc-review.googlesource.com/c/src/+/244083

Bug: webrtc:12517, webrtc:11993
Change-Id: I80d826a0d3f3240a2cecf8cbe09842cba0902b16
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/244085
Auto-Submit: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35611}

call/rtp_demuxer.h

diff --git a/call/rtp_demuxer.h b/call/rtp_demuxer.h
index fb65fce..931dac2 100644
--- a/call/rtp_demuxer.h
+++ b/call/rtp_demuxer.h
@@ -16,6 +16,7 @@
 #include <utility>
 #include <vector>
 
+#include "absl/strings/string_view.h"
 #include "rtc_base/containers/flat_map.h"
 #include "rtc_base/containers/flat_set.h"
 
@@ -27,6 +28,8 @@
 // This struct describes the criteria that will be used to match packets to a
 // specific sink.
 struct RtpDemuxerCriteria {
+  explicit RtpDemuxerCriteria(absl::string_view mid,
+                              absl::string_view rsid = absl::string_view());
   RtpDemuxerCriteria();
   ~RtpDemuxerCriteria();