[iOS] ChildBrowser: add .clear() method to flush CB history

In recent versions, browser history is retained across instances of ChildBrowser. Opening a second ChildBrowser, after closing a previous instance, results in the user being able to navigate back through the history of the previous CB instance.

For those who want a fresh, history-free experience in each CB instance, you can use the clear() method in the onClose callback:

```javascript
window.cb.onClose = function(){ window.cb.clear(); };
```
or by passing a boolean true to the close() method:

```javascript
window.cb.close(true);
```