A bug in the transpot feedback adapter causes new feedback message to
always start with a received packet. This makes it impossible for the
receiver to distinguish from actual dropped packets and dropped feedback
messages.
BUG=webrtc:6073
R=stefan@webrtc.org
Review URL: https://codereview.webrtc.org/2122863002 .
Cr-Commit-Position: refs/heads/master@{#13381}