Do not wrap a PCC sender with a PacingSender.

net/quic/core/quic_sent_packet_manager.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/quic/core/quic_sent_packet_manager.h"
 
 #include <algorithm>
 #include <string>
 
 #include "net/quic/chromium/quic_utils_chromium.h"
@@ skipping 104 matching lines @@
     } else {
       SetSendAlgorithm(kReno);
     }
   } else if (config.HasClientRequestedIndependentOption(kBYTE, perspective_)) {
     SetSendAlgorithm(kCubic);
   } else if (FLAGS_quic_reloadable_flag_quic_enable_pcc &&
              config.HasClientRequestedIndependentOption(kTPCC, perspective_)) {
     SetSendAlgorithm(kPCC);
   }
 
-  using_pacing_ = !FLAGS_quic_disable_pacing_for_perf_tests;
+  // The PCCSender implements its own version of pacing through the
+  // SendAlgorithm::TimeUntilSend() function.  Do not wrap a
+  // PacingSender around it, since wrapping a PacingSender around an
+  // already paced SendAlgorithm produces a DCHECK.  TODO(fayang):
+  // Change this if/when the PCCSender uses the PacingSender.
+  using_pacing_ = !FLAGS_quic_disable_pacing_for_perf_tests &&
+                  send_algorithm_->GetCongestionControlType() != kPCC;
 
   if (config.HasClientSentConnectionOption(k1CON, perspective_)) {
     send_algorithm_->SetNumEmulatedConnections(1);
   }
   if (config.HasClientSentConnectionOption(kNCON, perspective_)) {
     n_connection_simulation_ = true;
   }
   if (config.HasClientSentConnectionOption(kNTLP, perspective_)) {
     max_tail_loss_probes_ = 0;
   }
@@ skipping 814 matching lines @@
 
 void QuicSentPacketManager::OnApplicationLimited() {
   send_algorithm_->OnApplicationLimited(unacked_packets_.bytes_in_flight());
 }
 
 const SendAlgorithmInterface* QuicSentPacketManager::GetSendAlgorithm() const {
   return send_algorithm_.get();
 }
 
 }  // namespace net