Fix segfault when PeerConnection is destroyed during stats collection.

webrtc/api/rtcstatscollector.cc

Index: webrtc/api/rtcstatscollector.cc
diff --git a/webrtc/api/rtcstatscollector.cc b/webrtc/api/rtcstatscollector.cc
index a6d691c33339fd453ccf03175017e2422764e840..0cdff176b695c986f5c225a7179febfc72f8b2e3 100644
--- a/webrtc/api/rtcstatscollector.cc
+++ b/webrtc/api/rtcstatscollector.cc
@@ -389,6 +389,10 @@ RTCStatsCollector::RTCStatsCollector(PeerConnection* pc,
       this, &RTCStatsCollector::OnDataChannelCreated);
 }
 
+RTCStatsCollector::~RTCStatsCollector() {
+  RTC_DCHECK_EQ(num_pending_partial_reports_, 0);
+}
+
 void RTCStatsCollector::GetStatsReport(
     rtc::scoped_refptr<RTCStatsCollectorCallback> callback) {
   RTC_DCHECK(signaling_thread_->IsCurrent());
@@ -430,6 +434,17 @@ void RTCStatsCollector::ClearCachedStatsReport() {
   cached_report_ = nullptr;
 }
 
+void RTCStatsCollector::WaitForPendingRequest() {
+  RTC_DCHECK(signaling_thread_->IsCurrent());
+  if (num_pending_partial_reports_) {
+    rtc::Thread::Current()->ProcessMessages(0);
+    while (num_pending_partial_reports_) {
+      rtc::Thread::Current()->SleepMs(1);
+      rtc::Thread::Current()->ProcessMessages(0);
+    }
+  }
+}
+
 void RTCStatsCollector::ProducePartialResultsOnSignalingThread(
     int64_t timestamp_us) {
   RTC_DCHECK(signaling_thread_->IsCurrent());