indexedDB is null in Firefox

Hello! Just noticed this problem with greenlet on Firefox (66.0.3); indexedDB is `null` in the greenlet execution context.

Reproduction case:

```javascript
const callIndexedDB = greenlet(async (limit) => {
console.log(indexedDB);
// null
});
```

Also on Chrome (73.0.3683.103) it appears the success callback is never called:

```javascript
const callIndexedDB = greenlet(async (limit) => {
var db;
var request = indexedDB.open("MyTestDatabase");
request.onerror = function(event) {
alert("Why didn't you allow my web app to use IndexedDB?!");
};
request.onsuccess = function(event) {
console.log("Success!");
// Never called
};
});
```

Let me know if there's any other information I can provide that might be useful here.