Fix segfault when PeerConnection is destroyed during stats collection.

webrtc/api/rtcstatscollector.h

Index: webrtc/api/rtcstatscollector.h
diff --git a/webrtc/api/rtcstatscollector.h b/webrtc/api/rtcstatscollector.h
index c32d65f2460a83447946b70430b5bf1bc05b5074..3cf2b85311399992d3f5ad6b21cc16e735a3eb1e 100644
--- a/webrtc/api/rtcstatscollector.h
+++ b/webrtc/api/rtcstatscollector.h
@@ -70,6 +70,10 @@ class RTCStatsCollector : public virtual rtc::RefCountInterface,
   // calling |GetStatsReport| guarantees fresh stats.
   void ClearCachedStatsReport();
 
+  // If there is a |GetStatsReport| requests in-flight, waits until it has been
+  // completed. Must be called on the signaling thread.
+  void WaitForPendingRequest();
+
  protected:
   RTCStatsCollector(PeerConnection* pc, int64_t cache_lifetime_us);