Bug 1456672 - Add a brief note that we prefer to pass TimeStamp objects by value. r=heycam,kats DONTBUILD

MozReview-Commit-ID: 6EvYJvBv5DF

--HG--
extra : rebase_source : 054ca1491ebbdc4788f90bf9fc60da75130b9af8

mozglue/misc/TimeStamp.h

@@ -403,6 +403,9 @@ typedef BaseTimeDuration<TimeDurationValueCalculator> TimeDuration;
  * is initialized to the clock's epoch and provides a
  * time_since_epoch() method that functions similiarly. i.e.
  * t.IsNull() is equivalent to t.time_since_epoch() == decltype(t)::duration::zero();
+ *
+ * Note that, since TimeStamp objects are small, prefer to pass them by value
+ * unless there is a specific reason not to do so.
  */
 class TimeStamp
 {