Currently SMILAnimationFunction::ParseAttr only sets its aPreventCachingOfSandwich outparam when it needs to be set to true. (This lets us pass the same initially-false outparam into ParseAttr multiple times, and then check it for trueness at the end to see if any of the parsed values need us to prevent caching the sandwich.)
Our impls for ISMILAttr::ValueFromString should behave like that, too. Then, we can pass ParseAttr's outparam directly to ValueFromString.
Differential Revision: https://phabricator.services.mozilla.com/D174353
