CryptoKey global already practiced but not defined. #296

Every single browser implementation attaches the CryptoKey constructor to window or worker global scope. This is very useful for defensive programming when building libraries / code around web crypto, e.g. to fail early on invalid user input (when key is not an instanceof CryptoKey and wanting to offer a useful and consistent error message (unlike the implementation specific ones that often come with no information whatsoever).

I think https://w3c.github.io/webcrypto/#crypto-interface should get updated to reflect what's already the practice today and add CryptoKey alongside of the readonly crypto to WindowOrWorkerGlobalScope.