Propagate probing cluster id to SendTimeHistory.

webrtc/modules/pacing/bitrate_prober.cc

Index: webrtc/modules/pacing/bitrate_prober.cc
diff --git a/webrtc/modules/pacing/bitrate_prober.cc b/webrtc/modules/pacing/bitrate_prober.cc
index 8e8e36ea34be202017e81ff584bb9ec7b26e9079..19eaff7a69ad3fbce234bd3714b8f1756d87353f 100644
--- a/webrtc/modules/pacing/bitrate_prober.cc
+++ b/webrtc/modules/pacing/bitrate_prober.cc
@@ -154,6 +154,8 @@ void BitrateProber::PacketSent(int64_t now_ms, size_t packet_size) {
     ++cluster->sent_probe_packets;
     if (cluster->sent_probe_packets == cluster->max_probe_packets)
       clusters_.pop();
+    if (clusters_.empty())
+      probing_state_ = kWait;
   }
 }
 }  // namespace webrtc