Add a new overuse estimator for the delay based BWE behind experiment.

webrtc/modules/congestion_controller/BUILD.gn

Index: webrtc/modules/congestion_controller/BUILD.gn
diff --git a/webrtc/modules/congestion_controller/BUILD.gn b/webrtc/modules/congestion_controller/BUILD.gn
index e25c992a78346db9d4353ecf2b1bdf6c708df424..09b33881c84374081bf64e6a0752c1218ec9d6eb 100644
--- a/webrtc/modules/congestion_controller/BUILD.gn
+++ b/webrtc/modules/congestion_controller/BUILD.gn
@@ -22,8 +22,16 @@ rtc_static_library("congestion_controller") {
     "probing_interval_estimator.h",
     "transport_feedback_adapter.cc",
     "transport_feedback_adapter.h",
+    "trendline_estimator.cc",
+    "trendline_estimator.h",
   ]
 
+  if (rtc_enable_bwe_test_logging) {
+    defines = [ "BWE_TEST_LOGGING_COMPILE_TIME_ENABLE=1" ]
+  } else {
+    defines = [ "BWE_TEST_LOGGING_COMPILE_TIME_ENABLE=0" ]
+  }
+
   # TODO(jschuh): Bug 1348: fix this warning.
   configs += [ "//build/config/compiler:no_size_t_to_int_warning" ]