Add following helper methods to track successful QUIC client port migrations:

net/quic/quic_connection.cc

 // 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.
 
 #include "net/quic/quic_connection.h"
 
 #include <string.h>
 #include <sys/types.h>
 #include <algorithm>
 #include <iterator>
@@ skipping 1464 matching lines @@
     return false;
   }
 
   QuicPacketSequenceNumber sequence_number = pending_write_->sequence_number;
   TransmissionType transmission_type  = pending_write_->transmission_type;
   HasRetransmittableData retransmittable = pending_write_->retransmittable;
   size_t length = pending_write_->length;
   pending_write_.reset();
 
   if (result.status == WRITE_STATUS_ERROR) {
-    DVLOG(1) << "Write failed with error: " << result.error_code << " ("
-             << ErrorToString(result.error_code) << ")";
+    DVLOG(1) << ENDPOINT << "Write failed with error: " << result.error_code
+             << " (" << ErrorToString(result.error_code) << ")";
     // We can't send an error as the socket is presumably borked.
     CloseConnection(QUIC_PACKET_WRITE_ERROR, false);
     return false;
   }
 
   QuicTime now = clock_->Now();
   if (transmission_type == NOT_RETRANSMISSION) {
     time_of_last_sent_new_packet_ = now;
   }
   SetPingAlarm();
@@ skipping 480 matching lines @@
   // If we changed the generator's batch state, restore original batch state.
   if (!already_in_batch_mode_) {
     DVLOG(1) << "Leaving Batch Mode.";
     connection_->packet_generator_.FinishBatchOperations();
   }
   DCHECK_EQ(already_in_batch_mode_,
             connection_->packet_generator_.InBatchMode());
 }
 
 }  // namespace net