Adding the ability to use a simulated clock for unit tests.

This will be useful for any tests that test objects with time-dependent
behavior. It will allow such tests to be written in such a way that their
outcome is more repeatable (less flaky), and will also allow such tests
to finish quicker. For example, a test for STUN timeout doesn't need to
wait the full timeout interval in real time; it can simply advance the
simulated clock.

BUG=webrtc:4925
R=pthatcher@webrtc.org

Review URL: https://codereview.webrtc.org/1895933003 .

Cr-Commit-Position: refs/heads/master@{#12950}

webrtc/base/timing.h

@@ -13,6 +13,7 @@
 
 namespace rtc {
 
+// TODO(deadbeef): Remove this and use ClockInterface instead.
 class Timing {
  public:
   Timing();