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)
I didn't noticed this before, but this function should explicitly `return null;` at the end in case it falls out of the `for` loop (instead of implicitly returning `undefined`.).