Drop not awaited packets before decrypt them.

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 e070ef63fa2b54b9664c1a33a24372efd38172b4..c9707516f32cb8400aac4bbd26e223a02b799aeb 100644
--- a/net/quic/quic_connection.cc
+++ b/net/quic/quic_connection.cc
@@ -623,7 +623,8 @@ bool QuicConnection::OnUnauthenticatedHeader(const QuicPacketHeader& header) {
 
   // If this packet has already been seen, or the sender has told us that it
   // will not be retransmitted, then stop processing the packet.
-  if (!received_packet_manager_.IsAwaitingPacket(header.packet_number)) {
+  if (FLAGS_quic_drop_non_awaited_packets &&
+      !received_packet_manager_.IsAwaitingPacket(header.packet_number)) {
     DVLOG(1) << ENDPOINT << "Packet " << header.packet_number
              << " no longer being waited for.  Discarding.";
     if (debug_visitor_ != nullptr) {
@@ -1983,7 +1984,15 @@ void QuicConnection::MaybeProcessRevivedPacket() {
 
 QuicFecGroup* QuicConnection::GetFecGroup() {
   QuicFecGroupNumber fec_group_num = last_header_.fec_group;
-  if (fec_group_num == 0) {
+  if (fec_group_num == 0 ||
+      (FLAGS_quic_drop_non_awaited_packets &&
+       fec_group_num <
+           received_packet_manager_.peer_least_packet_awaiting_ack() &&
+       !ContainsKey(group_map_, fec_group_num))) {
+    // If the group number is below peer_least_packet_awaiting_ack and this
+    // group does not exist, which means this group has missing packets below
+    // |peer_least_packet_awaiting_ack| which we would never receive, so return
+    // nullptr.
     return nullptr;
   }
   if (!ContainsKey(group_map_, fec_group_num)) {
@@ -2078,9 +2087,9 @@ void QuicConnection::SendGoAway(QuicErrorCode error,
 void QuicConnection::CloseFecGroupsBefore(QuicPacketNumber packet_number) {
   FecGroupMap::iterator it = group_map_.begin();
   while (it != group_map_.end()) {
-    // If this is the current group or the group doesn't protect this packet
-    // we can ignore it.
-    if (last_header_.fec_group == it->first ||
+    // If the group doesn't protect this packet we can ignore it.
+    if ((!FLAGS_quic_drop_non_awaited_packets &&
+         last_header_.fec_group == it->first) ||
         !it->second->IsWaitingForPacketBefore(packet_number)) {
       ++it;
       continue;