TransportFeedback must be able to start with dropped packets.

webrtc/modules/rtp_rtcp/source/rtcp_packet/transport_feedback.cc

 /*
  *  Copyright (c) 2015 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 299 matching lines @@
 }
 
 uint32_t TransportFeedback::GetMediaSourceSsrc() const {
   return media_source_ssrc_;
 }
 void TransportFeedback::WithBase(uint16_t base_sequence,
                                  int64_t ref_timestamp_us) {
   RTC_DCHECK_EQ(-1, base_seq_);
   RTC_DCHECK_NE(-1, ref_timestamp_us);
   base_seq_ = base_sequence;
-  last_seq_ = base_sequence;
+  // last_seq_ is the sequence number of the last packed added _before_ a call
+  // to WithReceivedPacket(). Since the first sequence to be added is
+  // base_sequence, we need this to be one lower in order for potential missing
+  // packets to be populated properly.
+  last_seq_ = base_sequence - 1;
   base_time_ = ref_timestamp_us / kBaseScaleFactor;
   last_timestamp_ = base_time_ * kBaseScaleFactor;
 }
 
 void TransportFeedback::WithFeedbackSequenceNumber(uint8_t feedback_sequence) {
   feedback_seq_ = feedback_sequence;
 }
 
 bool TransportFeedback::WithReceivedPacket(uint16_t sequence_number,
                                            int64_t timestamp) {
@@ skipping 436 matching lines @@
                        "RLE block of size " << rle_chunk->NumSymbols()
                     << " but only " << max_size << " left to read.";
     delete rle_chunk;
     return nullptr;
   }
   return rle_chunk;
 }
 
 }  // namespace rtcp
 }  // namespace webrtc