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

webrtc/base/fakeclock.cc

Index: webrtc/base/fakeclock.cc
diff --git a/webrtc/base/fakeclock.cc b/webrtc/base/fakeclock.cc
index bcd720ff742f2687f6c0bb05c5e60672edec6314..d95408f6a8b18ed262a943a05af8bc133d13bad4 100644
--- a/webrtc/base/fakeclock.cc
+++ b/webrtc/base/fakeclock.cc
@@ -15,12 +15,12 @@
 
 namespace rtc {
 
-uint64_t FakeClock::TimeNanos() const {
+int64_t FakeClock::TimeNanos() const {
   CritScope cs(&lock_);
   return time_;
 }
 
-void FakeClock::SetTimeNanos(uint64_t nanos) {
+void FakeClock::SetTimeNanos(int64_t nanos) {
   {
     CritScope cs(&lock_);
     RTC_DCHECK(nanos >= time_);