Fix Live CSS with an iframe element

For #7935 and #7785 

Bugs to fix:
- [x] Only updating the iframe if the same CSS file is used for both iframe and page
- [ ] Changes to the iframe's HTML will only take effect on save
- [ ] Error message on startup (in console)
``` JS
* @param {?string} new content of every stylesheet. Defaults to doc.getText() if omitted
```
Is this valid then?