Avoid entering ProbeRTT when the connection is app limited

Summary: When restarting from idle, the probeRTT min value can be updated without having to enter ProbeRTT.

Reviewed By: mjoras

Differential Revision: D49548502

fbshipit-source-id: 8471df3e15eb788b087764f2630537c71e164914

quic/congestion_control/Bbr2.cpp

@@ -666,6 +666,10 @@ void Bbr2CongestionController::probeInflightHiUpward(uint64_t ackedBytes) {
 }
 
 void Bbr2CongestionController::updateMinRtt() {
+  if (idleRestart_) {
+    probeRttMinTimestamp_ = Clock::now();
+    probeRttMinValue_ = kDefaultMinRtt;
+  }
   probeRttExpired_ = probeRttMinTimestamp_
       ? Clock::now() > (probeRttMinTimestamp_.value() + kProbeRTTInterval)
       : true;