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

webrtc/api/statscollector.cc

Index: webrtc/api/statscollector.cc
diff --git a/webrtc/api/statscollector.cc b/webrtc/api/statscollector.cc
index a40835c77e257cac26fe7a0c5b8132c561b00972..7dc17da1595f7fc3ac968bdb1909d7e6fff5c37f 100644
--- a/webrtc/api/statscollector.cc
+++ b/webrtc/api/statscollector.cc
@@ -549,6 +549,11 @@ StatsReport* StatsCollector::PrepareReport(
   return report;
 }
 
+bool StatsCollector::IsValidTrack(const std::string& track_id) {
+  return reports_.Find(StatsReport::NewTypedId(
+             StatsReport::kStatsReportTypeTrack, track_id)) != nullptr;
+}
+
 StatsReport* StatsCollector::AddCertificateReports(
     const rtc::SSLCertificate* cert) {
   RTC_DCHECK(pc_->session()->signaling_thread()->IsCurrent());