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)
Note: I am getting the following exceptions just launching Live Preview in [this recipe](https://github.com/adobe/brackets/pull/8144#issuecomment-51273365):

```
Assertion failed: Attempted to call remote method without objectId set. RemoteAgent.js:58
_call RemoteAgent.js:58
Some arguments of method 'Runtime.callFunctionOn' can't be processed
Parameter 'objectId' with type 'String' was not found. 
Object
 
Object
 ErrorNotification.js:117
window.console.error ErrorNotification.js:117
```