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}
