Since the interval between the timestamps does not include the send/receive
time of the last/first packet we correct the interval by adding the average
of the interval between probing packets.
BUG=webrtc:5859
R=mflodman@webrtc.org, stefan@webrtc.org
Review URL: https://codereview.webrtc.org/2224173003 .
Cr-Commit-Position: refs/heads/master@{#13719}