A `<video>` element with no source won't have a natural aspect ratio,
but `aspect-ratio: auto` should still fall back to a ratio of 300/150.
`used_size_as_if_inline_element_from_content_box_sizes()` was already
handling this, but other consumers of `preferred_aspect_ratio()` were
wrong. In particular, this resulted in a 0px wide inline-block:
```html
<div style="display: inline-block; border: solid">
<video style="height: 100px; background: cyan"></video>
</div>
```
So this patch moves the fallback into `preferred_aspect_ratio()`.
Signed-off-by: Oriol Brufau <obrufau@igalia.com>
851b125d4b