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

Also updated types close to call sites.

BUG=webrtc:6733

Review-Url: https://codereview.webrtc.org/2514553003
Cr-Commit-Position: refs/heads/master@{#15255}

webrtc/base/timeutils.h

@@ -34,7 +34,7 @@ static const int64_t kNumNanosecsPerMicrosec =
 class ClockInterface {
  public:
   virtual ~ClockInterface() {}
-  virtual uint64_t TimeNanos() const = 0;
+  virtual int64_t TimeNanos() const = 0;
 };
 
 // Sets the global source of time. This is useful mainly for unit tests.
@@ -55,7 +55,7 @@ ClockInterface* SetClockForTesting(ClockInterface* clock);
 
 // Returns the actual system time, even if a clock is set for testing.
 // Useful for timeouts while using a test clock, or for logging.
-uint64_t SystemTimeNanos();
+int64_t SystemTimeNanos();
 int64_t SystemTimeMillis();
 
 // Returns the current time in milliseconds in 32 bits.
@@ -69,10 +69,11 @@ inline int64_t Time() {
 }
 
 // Returns the current time in microseconds.
-uint64_t TimeMicros();
+int64_t TimeMicros();
 
 // Returns the current time in nanoseconds.
-uint64_t TimeNanos();
+int64_t TimeNanos();
+
 
 // Returns a future timestamp, 'elapsed' milliseconds from now.
 int64_t TimeAfter(int64_t elapsed);
@@ -88,7 +89,7 @@ inline int64_t TimeSince(int64_t earlier) {
 }
 
 // The number of milliseconds that will elapse between now and 'later'.
-inline int64_t TimeUntil(uint64_t later) {
+inline int64_t TimeUntil(int64_t later) {
   return later - TimeMillis();
 }