Change rtc::TimeNanos and rtc::TimeMicros return value from uint64_t to int64_t.

webrtc/stats/rtcstatsreport.cc

Index: webrtc/stats/rtcstatsreport.cc
diff --git a/webrtc/stats/rtcstatsreport.cc b/webrtc/stats/rtcstatsreport.cc
index 9e2244cecf848b888416b4dbb5af9c4a8101dca1..3dd6eff39fc43518f7d615cf3c67e5ba7af68913 100644
--- a/webrtc/stats/rtcstatsreport.cc
+++ b/webrtc/stats/rtcstatsreport.cc
@@ -57,12 +57,12 @@ bool RTCStatsReport::ConstIterator::operator!=(
 }
 
 rtc::scoped_refptr<RTCStatsReport> RTCStatsReport::Create(
-    uint64_t timestamp_us) {
+    int64_t timestamp_us) {
   return rtc::scoped_refptr<RTCStatsReport>(
       new rtc::RefCountedObject<RTCStatsReport>(timestamp_us));
 }
 
-RTCStatsReport::RTCStatsReport(uint64_t timestamp_us)
+RTCStatsReport::RTCStatsReport(int64_t timestamp_us)
     : timestamp_us_(timestamp_us) {
 }