Return false if PeerConnection::GetStats() is called on invalid tracks

webrtc/api/statscollector.h

Index: webrtc/api/statscollector.h
diff --git a/webrtc/api/statscollector.h b/webrtc/api/statscollector.h
index adbac44b8047b2c67f1351b16292c656f39976b6..7c198217e8dea797d393212a436556abdca60c6d 100644
--- a/webrtc/api/statscollector.h
+++ b/webrtc/api/statscollector.h
@@ -78,6 +78,9 @@ class StatsCollector {
                              const StatsReport::Id& transport_id,
                              StatsReport::Direction direction);
 
+  // A track is invalid if there is no report data for it.
+  bool IsValidTrack(const std::string& track_id);
+
   // Method used by the unittest to force a update of stats since UpdateStats()
   // that occur less than kMinGatherStatsPeriod number of ms apart will be
   // ignored.