Replace QUIC's PathMtuAckListener with a non-AckListener approach. Protected by FLAGS_quic_no_mtu_d…

net/quic/quic_connection.cc

Index: net/quic/quic_connection.cc
diff --git a/net/quic/quic_connection.cc b/net/quic/quic_connection.cc
index ba181f950b2d2f49542d2832a81d1716e12ed328..a9a38c5525d3ae6200298bfaa8992bbd78bffec4 100644
--- a/net/quic/quic_connection.cc
+++ b/net/quic/quic_connection.cc
@@ -318,6 +318,8 @@ QuicConnection::QuicConnection(QuicConnectionId connection_id,
   framer_.set_received_entropy_calculator(&received_packet_manager_);
   last_stop_waiting_frame_.least_unacked = 0;
   stats_.connection_creation_time = clock_->ApproximateNow();
+  // TODO(ianswett): Supply the NetworkChangeVisitor as a constructor argument
+  // and make it required non-null, because it's always used.
   sent_packet_manager_->SetNetworkChangeVisitor(this);
   // Allow the packet writer to potentially reduce the packet size to a value
   // even smaller than kDefaultMaxPacketSize.
@@ -1806,6 +1808,13 @@ void QuicConnection::OnPathDegrading() {
   visitor_->OnPathDegrading();
 }
 
+void QuicConnection::OnPathMtuIncreased(QuicPacketLength packet_size) {
+  DCHECK(FLAGS_quic_no_mtu_discovery_ack_listener);
+  if (packet_size > max_packet_length()) {
+    SetMaxPacketLength(packet_size);
+  }
+}
+
 void QuicConnection::OnHandshakeComplete() {
   sent_packet_manager_->SetHandshakeConfirmed();
   // The client should immediately ack the SHLO to confirm the handshake is
@@ -2371,19 +2380,21 @@ void QuicConnection::SendMtuDiscoveryPacket(QuicByteCount target_mtu) {
 
   // Send the probe.
   packet_generator_.GenerateMtuDiscoveryPacket(
-      target_mtu, last_mtu_discovery_ack_listener.get());
+      target_mtu, FLAGS_quic_no_mtu_discovery_ack_listener
+                      ? nullptr
+                      : last_mtu_discovery_ack_listener.get());
 }
 
 void QuicConnection::DiscoverMtu() {
   DCHECK(!mtu_discovery_alarm_->IsSet());
 
-  // Chcek if the MTU has been already increased.
+  // Check if the MTU has been already increased.
   if (mtu_discovery_target_ <= max_packet_length()) {
     return;
   }
 
-  // Schedule the next probe *before* sending the current one.  This is
-  // important, otherwise, when SendMtuDiscoveryPacket() is called,
+  // Calculate the packet number of the next probe *before* sending the current
+  // one.  Otherwise, when SendMtuDiscoveryPacket() is called,
   // MaybeSetMtuAlarm() will not realize that the probe has been just sent, and
   // will reschedule this probe again.
   packets_between_mtu_probes_ *= 2;