Added implementation of three classes in BBR,with unit-tests.

webrtc/modules/remote_bitrate_estimator/test/estimators/congestion_window.h

 /*
  *  Copyright (c) 2017 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.
  *
  */
 
 #ifndef WEBRTC_MODULES_REMOTE_BITRATE_ESTIMATOR_TEST_ESTIMATORS_CONGESTION_WINDOW_H_
 #define WEBRTC_MODULES_REMOTE_BITRATE_ESTIMATOR_TEST_ESTIMATORS_CONGESTION_WINDOW_H_
 
 #include "webrtc/modules/remote_bitrate_estimator/test/estimators/bbr.h"
 
+#include "webrtc/rtc_base/optional.h"
+
 namespace webrtc {
 namespace testing {
 namespace bwe {
 class CongestionWindow {
  public:
   CongestionWindow();
   ~CongestionWindow();
   int GetCongestionWindow(BbrBweSender::Mode mode,
                           int64_t bandwidth_estimate,
-                          int64_t min_rtt,
+                          rtc::Optional<int64_t> min_rtt,
                           float gain);
   int GetTargetCongestionWindow(int64_t bandwidth_estimate,
-                                int64_t min_rtt,
+                                rtc::Optional<int64_t> min_rtt,
                                 float gain);
   // Packet sent from sender, meaning it is inflight until we receive it and we
   // should add packet's size to data_inflight.
   void PacketSent(size_t sent_packet_size);
 
   // Ack was received by sender, meaning packet is no longer inflight.
   void AckReceived(size_t received_packet_size);
 
   size_t data_inflight() { return data_inflight_bytes_; }
 
  private:
   size_t data_inflight_bytes_;
 };
 }  // namespace bwe
 }  // namespace testing
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_REMOTE_BITRATE_ESTIMATOR_TEST_ESTIMATORS_CONGESTION_WINDOW_H_