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/stats/rtcstats_objects.cc b/webrtc/stats/rtcstats_objects.cc
index 1cfe85f..46962f2 100644
--- a/webrtc/stats/rtcstats_objects.cc
+++ b/webrtc/stats/rtcstats_objects.cc
@@ -15,13 +15,13 @@
 const char RTCPeerConnectionStats::kType[] = "peer-connection";
 
 RTCPeerConnectionStats::RTCPeerConnectionStats(
-    const std::string& id, double timestamp)
-    : RTCPeerConnectionStats(std::string(id), timestamp) {
+    const std::string& id, int64_t timestamp_us)
+    : RTCPeerConnectionStats(std::string(id), timestamp_us) {
 }
 
 RTCPeerConnectionStats::RTCPeerConnectionStats(
-    std::string&& id, double timestamp)
-    : RTCStats(std::move(id), timestamp),
+    std::string&& id, int64_t timestamp_us)
+    : RTCStats(std::move(id), timestamp_us),
       data_channels_opened("dataChannelsOpened"),
       data_channels_closed("dataChannelsClosed") {
 }