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

webrtc/modules/remote_bitrate_estimator/BUILD.gn

Index: webrtc/modules/remote_bitrate_estimator/BUILD.gn
diff --git a/webrtc/modules/remote_bitrate_estimator/BUILD.gn b/webrtc/modules/remote_bitrate_estimator/BUILD.gn
index 5a3afc65fd89433dbd0195408b171da473c7f406..36d3aa58e563323910d56c67fa70a2dbb072a7e2 100644
--- a/webrtc/modules/remote_bitrate_estimator/BUILD.gn
+++ b/webrtc/modules/remote_bitrate_estimator/BUILD.gn
@@ -74,9 +74,11 @@ if (rtc_include_tests) {
       "test/bwe_test_logging.h",
       "test/estimators/bbr.cc",
       "test/estimators/bbr.h",
+      "test/estimators/congestion_window.cc",
       "test/estimators/congestion_window.h",
       "test/estimators/max_bandwidth_filter.cc",
       "test/estimators/max_bandwidth_filter.h",
+      "test/estimators/min_rtt_filter.cc",
       "test/estimators/min_rtt_filter.h",
       "test/estimators/nada.cc",
       "test/estimators/nada.h",
@@ -177,6 +179,9 @@ if (rtc_include_tests) {
       "send_time_history_unittest.cc",
       "test/bwe_test_framework_unittest.cc",
       "test/bwe_unittest.cc",
+      "test/estimators/congestion_window_unittest.cc",
+      "test/estimators/max_bandwidth_filter_unittest.cc",
+      "test/estimators/min_rtt_filter_unittest.cc",
       "test/estimators/nada_unittest.cc",
       "test/metric_recorder_unittest.cc",
     ]