Adds support to the PacketDroppingTestWriter for dropping the first N

net/tools/quic/test_tools/packet_dropping_test_writer.cc

Index: net/tools/quic/test_tools/packet_dropping_test_writer.cc
diff --git a/net/tools/quic/test_tools/packet_dropping_test_writer.cc b/net/tools/quic/test_tools/packet_dropping_test_writer.cc
index e31a72826bbbf4de16353bac7135f4c5190b0af4..2ef0ba90bf5f6d8125511ba1eb451d85d2a87032 100644
--- a/net/tools/quic/test_tools/packet_dropping_test_writer.cc
+++ b/net/tools/quic/test_tools/packet_dropping_test_writer.cc
@@ -47,7 +47,9 @@ PacketDroppingTestWriter::PacketDroppingTestWriter()
     : clock_(nullptr),
       cur_buffer_size_(0),
       config_mutex_(),
+      num_calls_to_write_(0),
       fake_packet_loss_percentage_(0),
+      fake_drop_first_n_packets_(0),
       fake_blocked_socket_percentage_(0),
       fake_packet_reorder_percentage_(0),
       fake_packet_delay_(QuicTime::Delta::Zero()),
@@ -76,9 +78,16 @@ WriteResult PacketDroppingTestWriter::WritePacket(
     size_t buf_len,
     const net::IPAddressNumber& self_address,
     const net::IPEndPoint& peer_address) {
+  ++num_calls_to_write_;
   ReleaseOldPackets();
 
   base::AutoLock locked(config_mutex_);
+  if (fake_drop_first_n_packets_ > 0 &&
+      num_calls_to_write_ <= fake_drop_first_n_packets_) {
+    DVLOG(1) << "Dropping first " << fake_drop_first_n_packets_
+             << " packets (packet number " << num_calls_to_write_ << ")";
+    return WriteResult(WRITE_STATUS_OK, buf_len);
+  }
   if (fake_packet_loss_percentage_ > 0 &&
       simple_random_.RandUint64() % 100 <
           static_cast<uint64>(fake_packet_loss_percentage_)) {