This makes the sample time more "strongly typed" by using a variant-style
class instead of a raw TimeStamp. The sample time can come from different
sources (vsync, testing, Now()) and it is useful to save that information
as we pass around the sample time. It would have been helpful with some
of the debugging I did recently, and it also is needed for the next patch.
Differential Revision: https://phabricator.services.mozilla.com/D86244
Kartikaya Gupta