In this CL:
- Removed unused variable |last_seq_num_|.
- Fixed bug where a new incomplete frame was detected as a complete frame.
- Added fuzzer to video_coding::PacketBuffer.
BUG=chromium:677101
Review-Url: https://codereview.webrtc.org/2613833003
Cr-Commit-Position: refs/heads/master@{#16003}
