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}
diff --git a/webrtc/base/timeutils_unittest.cc b/webrtc/base/timeutils_unittest.cc
index 6707d8c..db939cb 100644
--- a/webrtc/base/timeutils_unittest.cc
+++ b/webrtc/base/timeutils_unittest.cc
@@ -303,11 +303,11 @@
   FakeClock clock;
   SetClockForTesting(&clock);
 
-  clock.SetTimeNanos(987654321u);
+  clock.SetTimeNanos(987654321);
   EXPECT_EQ(987u, Time32());
   EXPECT_EQ(987, TimeMillis());
-  EXPECT_EQ(987654u, TimeMicros());
-  EXPECT_EQ(987654321u, TimeNanos());
+  EXPECT_EQ(987654, TimeMicros());
+  EXPECT_EQ(987654321, TimeNanos());
   EXPECT_EQ(1000u, TimeAfter(13));
 
   SetClockForTesting(nullptr);
@@ -317,27 +317,27 @@
 
 TEST(FakeClock, InitialTime) {
   FakeClock clock;
-  EXPECT_EQ(0u, clock.TimeNanos());
+  EXPECT_EQ(0, clock.TimeNanos());
 }
 
 TEST(FakeClock, SetTimeNanos) {
   FakeClock clock;
-  clock.SetTimeNanos(123u);
-  EXPECT_EQ(123u, clock.TimeNanos());
-  clock.SetTimeNanos(456u);
-  EXPECT_EQ(456u, clock.TimeNanos());
+  clock.SetTimeNanos(123);
+  EXPECT_EQ(123, clock.TimeNanos());
+  clock.SetTimeNanos(456);
+  EXPECT_EQ(456, clock.TimeNanos());
 }
 
 TEST(FakeClock, AdvanceTime) {
   FakeClock clock;
   clock.AdvanceTime(TimeDelta::FromNanoseconds(1111u));
-  EXPECT_EQ(1111u, clock.TimeNanos());
+  EXPECT_EQ(1111, clock.TimeNanos());
   clock.AdvanceTime(TimeDelta::FromMicroseconds(2222u));
-  EXPECT_EQ(2223111u, clock.TimeNanos());
+  EXPECT_EQ(2223111, clock.TimeNanos());
   clock.AdvanceTime(TimeDelta::FromMilliseconds(3333u));
-  EXPECT_EQ(3335223111u, clock.TimeNanos());
+  EXPECT_EQ(3335223111, clock.TimeNanos());
   clock.AdvanceTime(TimeDelta::FromSeconds(4444u));
-  EXPECT_EQ(4447335223111u, clock.TimeNanos());
+  EXPECT_EQ(4447335223111, clock.TimeNanos());
 }
 
 // When the clock is advanced, threads that are waiting in a socket select