The nsISimpleEnumerator contract specifies that GetNext() returns
NS_ERROR_FAILURE when iteration is complete. Several implementations, however,
either return NS_OK and a null result, or return some other error code, when
iteration is complete.
Since my initial implementation of the JS iteration stubs rely on the
contract-defined behavior of GetNext(), these need to be fixed before it can
land.
Differential Revision: https://phabricator.services.mozilla.com/D3726
09aa6cec83