Real time clock may cause problems as they can move (even backwards) if
the clock is changed, eg updated by NTP.
Non-monotonic clocks still in use on some platform (I'm looking at you,
Apple) for timed waits, but that should be less of an issue than actual
timestamps.
BUG=webrtc:5452
Review URL: https://codereview.webrtc.org/1613013002
Cr-Commit-Position: refs/heads/master@{#11375}
e791ffd638