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

When the fake clock's time is advanced, it now ensures all pending
queued messages have been dispatched. This allows us to write a
"SIMULATED_WAIT" macro that ticks the simulated clock by milliseconds up
until the target time.

Useful in this case, where we know the STUN timeout should take a total
of 9500ms, but it would be overly complex to write test code that waits
for each individual timeout, ensures a STUN packet has been
retransmited, etc.

(The test described above *should* be written, but it belongs in
p2ptransportchannel_unittest.cc, not webrtcsession_unittest.cc).

Review-Url: https://codereview.webrtc.org/2024813004
Cr-Commit-Position: refs/heads/master@{#13052}
diff --git a/webrtc/base/timeutils.cc b/webrtc/base/timeutils.cc
index ecd0911..3c89d80 100644
--- a/webrtc/base/timeutils.cc
+++ b/webrtc/base/timeutils.cc
@@ -32,8 +32,10 @@
 
 ClockInterface* g_clock = nullptr;
 
-void SetClock(ClockInterface* clock) {
+ClockInterface* SetClockForTesting(ClockInterface* clock) {
+  ClockInterface* prev = g_clock;
   g_clock = clock;
+  return prev;
 }
 
 uint64_t TimeNanos() {