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);

[Taylor Brandstetter, 2016/12/16 18:53:27]

nit: Could use an rtc::Event that's set when a partial stats report completes
instead of SleepMs.

[hbos, 2016/12/19 09:52:48]

rtc::Event's wait does not process messages. In order for
num_pending_partial_reports_ to decrease we have to do message processing (doe
to jumping to other threads for stats collecting and back to the signaling
thread for the stats report merging and invoking callback).

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