Implemented ScopedFakeTime, faking rtc::TimeNanos.

webrtc/base/timeutils.cc

Index: webrtc/base/timeutils.cc
diff --git a/webrtc/base/timeutils.cc b/webrtc/base/timeutils.cc
index a9fe49d6075a2341a387247ccb8e941677ee3544..fedbe85e0caa161330290797c5452ac7ed02ef36 100644
--- a/webrtc/base/timeutils.cc
+++ b/webrtc/base/timeutils.cc
@@ -30,7 +30,26 @@
 
 namespace rtc {
 
+namespace test {
+
+namespace {
+FakeTimeInterface* g_fake_time = nullptr;
+}  // Anonymous
+
+void SetFakeTime(FakeTimeInterface* fake_time) {
+  // Allow setting g_fake_time non-null only if old value is null.
+  if (fake_time)
+    RTC_CHECK(!g_fake_time);
+
+  g_fake_time = fake_time;
+}
+
+}  // namespace test
+
 uint64_t TimeNanos() {
+  if (test::g_fake_time)
+    return test::g_fake_time->TimeNanos();
+
   int64_t ticks = 0;
 #if defined(WEBRTC_MAC)
   static mach_timebase_info_data_t timebase;