This saves having to iterate trough all packets in flight to compute the number of outstanding bytes. Bug: webrtc:8415 Change-Id: I35b135f37649a38b44a36d300af42a815f85192d Reviewed-on: https://webrtc-review.googlesource.com/77727 Commit-Queue: Sebastian Jansson <srte@webrtc.org> Reviewed-by: Björn Terelius <terelius@webrtc.org> Cr-Commit-Position: refs/heads/master@{#23398}