Fix segfault when PeerConnection is destroyed during stats collection.

webrtc/api/peerconnection.cc

Index: webrtc/api/peerconnection.cc
diff --git a/webrtc/api/peerconnection.cc b/webrtc/api/peerconnection.cc
index ebce40ca5eb0cf37d03243663826ea911eda06c3..58174f08c11bbd10d36e58c3420a3d06ff8b909e 100644
--- a/webrtc/api/peerconnection.cc
+++ b/webrtc/api/peerconnection.cc
@@ -608,6 +608,10 @@ PeerConnection::~PeerConnection() {
   }
   // Destroy stats_ because it depends on session_.
   stats_.reset(nullptr);
+  if (stats_collector_) {
+    stats_collector_->WaitForPendingRequest();
+    stats_collector_ = nullptr;
+  }
   // Now destroy session_ before destroying other members,
   // because its destruction fires signals (such as VoiceChannelDestroyed)
   // which will trigger some final actions in PeerConnection...