Probing: Add support for exponential startup probing

Adds support for exponentially probing the bandwidth at start-up to allow
ramp-up to real capacity of the network.

BUG=webrtc:6332
R=philipel@webrtc.org, stefan@webrtc.org

Review URL: https://codereview.webrtc.org/2235373004 .

Cr-Commit-Position: refs/heads/master@{#14189}
diff --git a/webrtc/modules/bitrate_controller/bitrate_controller_unittest.cc b/webrtc/modules/bitrate_controller/bitrate_controller_unittest.cc
index a70eecf..bfc44ea 100644
--- a/webrtc/modules/bitrate_controller/bitrate_controller_unittest.cc
+++ b/webrtc/modules/bitrate_controller/bitrate_controller_unittest.cc
@@ -170,7 +170,7 @@
   EXPECT_EQ(300000, bitrate_observer_.last_bitrate_);
 
   // Test that a low delay-based estimate limits the combined estimate.
-  controller_->UpdateDelayBasedEstimate(280000);
+  controller_->OnReceiveBitrateChanged({0}, 280000);
   EXPECT_EQ(280000, bitrate_observer_.last_bitrate_);
 
   // Test that a low REMB limits the combined estimate.