waitForNavigations

For sites that use `history.pushState`, they might do something like

```js
history.pushState(null, null, 'http://a');
// wait
history.pushState(null, null, 'http://b');
// wait
history.pushState(null, null, 'http://a');
// done
```

We can't use waitForFunction to check if the url is `a`, because then we might stop before the whole sequence has finished.