Now probe for x3 and x6 the initial start bitrate.

webrtc/modules/congestion_controller/probe_bitrate_estimator.cc

 /*
  *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #include "webrtc/modules/congestion_controller/probe_bitrate_estimator.h"
 
 #include <algorithm>
 
 #include "webrtc/base/logging.h"
 
 namespace {
 // Max number of saved clusters.
 constexpr size_t kMaxNumSavedClusters = 5;
 
 // The minumum number of probes we need for a valid cluster.
 constexpr int kMinNumProbesValidCluster = 4;
 
 // The maximum (receive rate)/(send rate) ratio for a valid estimate.
-constexpr float kValidRatio = 1.2f;
+constexpr float kValidRatio = 2.0f;
 }
 
 namespace webrtc {
 
 ProbingResult::ProbingResult() : bps(kNoEstimate), timestamp(0) {}
 
 ProbingResult::ProbingResult(int bps, int64_t timestamp)
     : bps(bps), timestamp(timestamp) {}
 
 bool ProbingResult::valid() const {
@@ skipping 75 matching lines @@
                << " [cluster id: " << packet_info.probe_cluster_id
                << "] [send: " << cluster->size << " bytes / "
                << send_interval_ms << " ms = " << send_bps / 1000 << " kb/s]"
                << " [receive: " << cluster->size << " bytes / "
                << receive_interval_ms << " ms = " << receive_bps / 1000
                << " kb/s]";
 
   return ProbingResult(result_bps, packet_info.arrival_time_ms);
 }
 }  // namespace webrtc