Fix a newly introduced bug in QUIC's PacingSender where burst_tokens_

net/quic/congestion_control/pacing_sender.h

 // Copyright (c) 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.
 //
 // A send algorithm which adds pacing on top of an another send algorithm.
 // It uses the underlying sender's bandwidth estimate to determine the
 // pacing rate to be used.  It also takes into consideration the expected
 // resolution of the underlying alarm mechanism to ensure that alarms are
 // not set too aggressively, and to smooth out variations.
 
@@ skipping 45 matching lines @@
       HasRetransmittableData has_retransmittable_data) const OVERRIDE;
   virtual QuicBandwidth BandwidthEstimate() const OVERRIDE;
   virtual bool HasReliableBandwidthEstimate() const OVERRIDE;
   virtual QuicTime::Delta RetransmissionDelay() const OVERRIDE;
   virtual QuicByteCount GetCongestionWindow() const OVERRIDE;
 
  private:
   scoped_ptr<SendAlgorithmInterface> sender_;  // Underlying sender.
   QuicTime::Delta alarm_granularity_;
   uint32 initial_packet_burst_;
-  uint32 burst_tokens_;
+  mutable uint32 burst_tokens_;
   // Send time of the last packet considered delayed.
   QuicTime last_delayed_packet_sent_time_;
   QuicTime next_packet_send_time_;  // When can the next packet be sent.
   mutable bool was_last_send_delayed_;  // True when the last send was delayed.
   bool has_valid_rtt_;  // True if we have at least one RTT update.
 
   DISALLOW_COPY_AND_ASSIGN(PacingSender);
 };
 
 }  // namespace net
 
 #endif  // NET_QUIC_CONGESTION_CONTROL_PACING_SENDER_H_