Content box query includes non-content box sizes

Testcase:
```html
<!doctype html>
<body>
<img src="https://www.joshmatthews.net/portrait.jpg" style="margin: 10px; padding: 10px; border: 10px solid; height: 10px; width:10px">
<script>console.log(document.body.firstElementChild.width)</script>
```

In Firefox this prints "10" as expected. In Servo it prints "70".