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..f4a1604f9df70e613aa226d02d8bf2f843b8007d 100644
--- a/webrtc/api/rtcstatscollector.cc
+++ b/webrtc/api/rtcstatscollector.cc
@@ -430,6 +430,17 @@ 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);

[hta-webrtc, 2016/12/16 14:05:31]

This is a busy-wait. Is it possible to wait on something asynchronous instead
(like a callback)?

[hbos, 2016/12/16 14:34:52]

We need to ProcessMessages (e.g. ProducePartialResultsOnSignalingThread or
AddPartialResults_s or if any other part of our code base that might get
executed posts to the signaling thread) and we need to do it blockingly,
otherwise the PeerConnection destructor would proceed to delete the resources
that we can't delete until we wait.

+      rtc::Thread::Current()->ProcessMessages(0);
+    }
+  }
+}
+
 void RTCStatsCollector::ProducePartialResultsOnSignalingThread(
     int64_t timestamp_us) {
   RTC_DCHECK(signaling_thread_->IsCurrent());