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

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

Index: webrtc/modules/remote_bitrate_estimator/test/estimators/congestion_window.h
diff --git a/webrtc/modules/remote_bitrate_estimator/test/estimators/congestion_window.h b/webrtc/modules/remote_bitrate_estimator/test/estimators/congestion_window.h
index 4fc1a125481e562a48dc9f32fdd005c3b9bfe078..105a748d63e8a857a0d3e6a629e9212b1c3aa57c 100644
--- a/webrtc/modules/remote_bitrate_estimator/test/estimators/congestion_window.h
+++ b/webrtc/modules/remote_bitrate_estimator/test/estimators/congestion_window.h
@@ -14,6 +14,8 @@
 
 #include "webrtc/modules/remote_bitrate_estimator/test/estimators/bbr.h"
 
+#include "webrtc/rtc_base/optional.h"
+
 namespace webrtc {
 namespace testing {
 namespace bwe {
@@ -23,10 +25,10 @@ class 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.