For QUIC BBRv2 STARTUP, factor out two pre-conditions to check bandwidth growth and losses: !full_bandwidth_reached_ && congestion_event.end_of_round_trip. The goal is to make cl/336154008 cleaner and easier to review. PiperOrigin-RevId: 336558917 Change-Id: Ie03bab3f895035e6803eac43f4b684611f9ff140