When constructing an Animation a null should be distinguished
from nothing being passed at all. This is not achieved by
use of our custom ExplicitNull IDL extended attribute.
Instead use the argument count to determine if the argument is
missing, which is a much more common pattern in the codebase.
In level 2 of the web animations spec, times are no longer always
measures in milliseconds, they can also be percents when dealing with
progress-based (i.e. scroll-based) timelines.
We don't actually support percent times yet but this change will make it
easier to implement when we do.
