`keydown` event doesn't fire for the Tab key

**Describe the bug:**
See title.

**To Reproduce:**
```html
<!doctype html>
<h2 id='Z'></h2>
<script>
document.addEventListener(`keydown`, e => {
  Z.innerText = `You pressed: ${e.key}`
})
</script>
```
Press tab. On Firefox it will trigger the listener but not Servo.

**Platform:**
Win10
