Do not wrap a PCC sender with a PacingSender.

net/quic/core/quic_sent_packet_manager.cc

Index: net/quic/core/quic_sent_packet_manager.cc
diff --git a/net/quic/core/quic_sent_packet_manager.cc b/net/quic/core/quic_sent_packet_manager.cc
index c71b137e25aa1687d9ce12e21f2aead4a5230b8c..9b3b3dc9b9434435623297f20c30076fa30bfc60 100644
--- a/net/quic/core/quic_sent_packet_manager.cc
+++ b/net/quic/core/quic_sent_packet_manager.cc
@@ -122,7 +122,13 @@ void QuicSentPacketManager::SetFromConfig(const QuicConfig& config) {
     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);