This doesn't cause issues now because responsive images over-invalidate. But I'm fixing that in bug 1076583, and without this change we'd fail some tests, because we look at the wrong resolution. It makes sense to only notify of MediaFeatureValuesChanged after our state is set, so that the new state can be properly inspected. All other callers already do that. Differential Revision: https://phabricator.services.mozilla.com/D218058