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-Commit-Position: refs/heads/master@{#13997}
diff --git a/webrtc/api/rtcstats_objects.h b/webrtc/api/rtcstats_objects.h
index 8c03edf..4bc889d 100644
--- a/webrtc/api/rtcstats_objects.h
+++ b/webrtc/api/rtcstats_objects.h
@@ -19,8 +19,8 @@
 
 class RTCPeerConnectionStats : public RTCStats {
  public:
-  RTCPeerConnectionStats(const std::string& id, double timestamp);
-  RTCPeerConnectionStats(std::string&& id, double timestamp);
+  RTCPeerConnectionStats(const std::string& id, int64_t timestamp_us);
+  RTCPeerConnectionStats(std::string&& id, int64_t timestamp_us);
 
   WEBRTC_RTCSTATS_IMPL(RTCStats, RTCPeerConnectionStats,
       &data_channels_opened,