Shady Parts: don't call _applyStyleProperties twice

Currently, when an instance consumes a ::part rule that consumes a custom property, and native custom properties are not available, `_applyStyleProperties` ends up getting run twice. Once is the normal case, via `styleElement` when the instance is connected. Then again, after the DOM has stamped and we notice that there are part rules to handle.

It might make sense to skip the first `_applyStyleProperties` if we know that an instance might be receiving a part rule with a custom property.