Improving the fake clock and using it to fix a flaky STUN timeout test.

webrtc/base/timeutils.cc

Index: webrtc/base/timeutils.cc
diff --git a/webrtc/base/timeutils.cc b/webrtc/base/timeutils.cc
index ecd0911a510dde654177bb368108d189ccc04645..3c89d808b39f97f6e2a8ace502c04d769269670d 100644
--- a/webrtc/base/timeutils.cc
+++ b/webrtc/base/timeutils.cc
@@ -32,8 +32,10 @@ namespace rtc {
 
 ClockInterface* g_clock = nullptr;
 
-void SetClock(ClockInterface* clock) {
+ClockInterface* SetClockForTesting(ClockInterface* clock) {
+  ClockInterface* prev = g_clock;
   g_clock = clock;
+  return prev;
 }
 
 uint64_t TimeNanos() {