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 (maybe FOL)
- [ ] Error message on startup (in console)
I don't see that error, but it seems like it's a race condition. The `_objectId` is maintained internally by RemoteAgent, and the only change is a return that only fires for iframes.