Set bandwidthHi when entering Drain

Summary:
When BBR2 switches from Startup to Drain, it starts applying a limit on pacing using bandwidthHi_. If this transition happens around an idleRestart, populating bandwidthHi_ from bandwidthLatest only, can result in lower samples that underutilize the link.

This change ensures that transitioning from Startup to Drain maintains the pacing rate.

Reviewed By: mjoras

Differential Revision: D49550971

fbshipit-source-id: 8f2644e8636382bc3498b3ec9c82e98cd15aa6d2

quic/congestion_control/Bbr2.cpp

@@ -429,6 +429,7 @@ void Bbr2CongestionController::checkStartupDone() {
   checkStartupHighLoss();
 
   if (state_ == State::Startup && filledPipe_) {
+    bandwidthHi_ = maxBwFilter_.GetBest();
     enterDrain();
   }
 }