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}