Properly handle fallback aspect ratio for videos (#34082)

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>

tests/wpt/meta/MANIFEST.json

@@ -570555,6 +570555,13 @@
        {}
       ]
      ],
+     "intrinsic-size-fallback-video.html": [
+      "c9f2a0539147524c7e4945c6ea2ef0f05b07323f",
+      [
+       null,
+       {}
+      ]
+     ],
      "keyword-sizes-for-intrinsic-contributions.tentative.html": [
       "5f135d8181b023134eb24e5ec34957e3a25e6765",
       [