Only set one QUIC sending alarm at once. Flag protected by --quic_only_one_sending_alarm.

net/quic/quic_flags.cc

Index: net/quic/quic_flags.cc
diff --git a/net/quic/quic_flags.cc b/net/quic/quic_flags.cc
index 0f02ed6bfc401e2216c3e1f9933ed344fee782e8..f2f7bcd6725d3a29ab9b316d59c470effd06c273 100644
--- a/net/quic/quic_flags.cc
+++ b/net/quic/quic_flags.cc
@@ -125,3 +125,7 @@ bool FLAGS_quic_enable_autotune_by_default = true;
 // Use largest acked in the most recent ack instead of largest acked ever in
 // loss recovery.
 bool FLAGS_quic_loss_recovery_use_largest_acked = true;
+
+// Only set one alarm for sending at once, either the send alarm or
+// retransmission alarm.
+bool FLAGS_quic_only_one_sending_alarm = true;