zero is TimeBase's is_null() so almost all code assumes TickClocks don't return zero. As a result, the majority of STTC usage requires two or three lines of initialization, e.g.,:

  // Fake clock. Make sure start time is non zero.
  base::SimpleTestTickClock testing_clock;
  testing_clock.Advance(base::TimeDelta::FromSeconds(1));

A bunch of tests fail if we do this (https://codereview.chromium.org/2462773002/) so we should either, a) fix them all, or b) add a static factory function, e.g., SimpleTestTickClock* NewNonNullSimpleTestTickClock().

WDYT phajdan.jr@?