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)
Yes, that's the big problem we had.
Let's say you've got this code:
``` HTML
<html>
  <head>
    <link rel="stylesheet" href="css/style.css" />
  </head>
  <body>
    <iframe>
      <html>
        <head>
          <link rel="stylesheet" href="css/style.css" />
        </head>
        <body>
          Foo.
        <body>
      </html>
    </iframe>
  </body>
</html>
```
In that case, you've got the exact same stylesheet used in two different documents on one page, and what this means is that the browser/inspector has two different stylesheet objects with the same URL.