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.

Start by running the supplied greenlet reproduction in Firefox 66.0.3 and Chrome 73.0.3683.103, then inspect the greenlet execution context and the indexedDB.open success callback. Done means the browser-specific null value and missing callback are explained and a project-supported resolution is identified.

Written by the indexing model from the issue text.