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..cd8ee0ff471963809fb1be4e54ca49abd80e603b 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 are any |GetStatsReport| requests in-flight, waits until it has

[hbos, 2016/12/16 13:49:35]

nit: Will update comment to speak in singular, this makes it sound like there
could be multiple concurrent requests, but there can only be one at a time.
(Multiple GetStatsReport calls are piggybacked)

+  // been completed. Must be called on the signaling thread.
+  void WaitForPendingRequest();
+
  protected:
   RTCStatsCollector(PeerConnection* pc, int64_t cache_lifetime_us);