[cssom] Can't tell if a StyleSheet has loaded

[dbaron]

The definition of StyleSheet (or maybe it belongs in CSSStyleSheet, but I suspect not) doesn't provide a mechanism to tell if the sheet is loaded. You can get a load event when it loads, but you can't tell whether that event has already happened, via, e.g., a readyState or complete property.

This issue came from:
https://twitter.com/cramforce/status/792588294940790784
https://twitter.com/BrendanEich/status/792791949673504768
https://twitter.com/cramforce/status/792793340802584576