Protect against the divide by zero error in QuicBandwidth::TransferTime.

net/quic/congestion_control/send_algorithm_interface.h

 // Copyright (c) 2012 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.
 //
 // The pure virtual class for send side congestion control algorithm.
 
 #ifndef NET_QUIC_CONGESTION_CONTROL_SEND_ALGORITHM_INTERFACE_H_
 #define NET_QUIC_CONGESTION_CONTROL_SEND_ALGORITHM_INTERFACE_H_
 
 #include <algorithm>
@@ skipping 64 matching lines @@
 
   // Called when the last retransmission timeout was spurious.
   virtual void RevertRetransmissionTimeout() = 0;
 
   // Calculate the time until we can send the next packet.
   virtual QuicTime::Delta TimeUntilSend(
       QuicTime now,
       QuicByteCount bytes_in_flight,
       HasRetransmittableData has_retransmittable_data) const = 0;
 
+  // The pacing rate of the send algorithm.  May be zero if the rate is unknown.
   virtual QuicBandwidth PacingRate() const = 0;
 
   // What's the current estimated bandwidth in bytes per second.
   // Returns 0 when it does not have an estimate.
   virtual QuicBandwidth BandwidthEstimate() const = 0;
 
   // Returns true if the current bandwidth estimate is reliable.
   virtual bool HasReliableBandwidthEstimate() const = 0;
 
   // Get the send algorithm specific retransmission delay, called RTO in TCP,
@@ skipping 17 matching lines @@
   // aka ssthresh.  Some send algorithms do not define a slow start
   // threshold and will return 0.
   virtual QuicByteCount GetSlowStartThreshold() const = 0;
 
   virtual CongestionControlType GetCongestionControlType() const = 0;
 };
 
 }  // namespace net
 
 #endif  // NET_QUIC_CONGESTION_CONTROL_SEND_ALGORITHM_INTERFACE_H_