Add AddPhasedLoop to support phased loops

This lets us convert phased loops over nicely in various places.

Change-Id: Icdde4520f991fc541fbbe7ab3d9b945bb8c12e83
diff --git a/aos/events/simulated-event-loop_test.cc b/aos/events/simulated-event-loop_test.cc
index bcc6f28..be52243 100644
--- a/aos/events/simulated-event-loop_test.cc
+++ b/aos/events/simulated-event-loop_test.cc
@@ -18,6 +18,10 @@
 
   void Run() override { event_loop_factory_.Run(); }
 
+  // TODO(austin): Implement this.  It's used currently for a phased loop test.
+  // I'm not sure how much that matters.
+  void SleepFor(::std::chrono::nanoseconds /*duration*/) override {}
+
  private:
    SimulatedEventLoopFactory event_loop_factory_;
 };