Using a timestamp based on a timer that is monotonically increasing for
the cache, so that cache's freshness can be checked regardless of if
system clock is modified.
Using a system clock for the stats' timestamp, which needs to be
relative to UNIX epoch (Jan 1, 1970, UTC).
This CL removes the dependency on faketiming.h.
BUG=chromium:627816
NOTRY=True
Review-Url: https://codereview.webrtc.org/2299643002
Cr-Commit-Position: refs/heads/master@{#13997}
0e6758d7ef