RTCStatsCollector: timestamps updated.

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-Original-Commit-Position: refs/heads/master@{#13997}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 0e6758d7efc64a93d27f45b278d867d7f13f2131
9 files changed