Add DVQA support for scenarios with new participants joining

Bug: webrtc:12247
Change-Id: Id51a2ab34e0b802e11931cad13f48ce8eefddcae
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/196361
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Andrey Logvin <landrey@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32804}

api/test/video_quality_analyzer_interface.h

@@ -138,6 +138,9 @@ class VideoQualityAnalyzerInterface : public StatsObserverInterface {
       absl::string_view pc_label,
       const rtc::scoped_refptr<const RTCStatsReport>& report) override {}
 
+  // Will be called before test adds new participant in the middle of a call.
+  virtual void RegisterParticipantInCall(absl::string_view peer_name) {}
+
   // Tells analyzer that analysis complete and it should calculate final
   // statistics.
   virtual void Stop() {}