layout: Support content: <image> on non-pseudo-elements

[servo-wpt-sync]

Per CSS-CONTENT-3, one of the possible values of the content CSS property is <content-replacement>, which evaluates to a single <image>. This value is also allowed on regular elements, not just on pseudo-elements, and it will make the element into a replaced element representing the given image, discarding its contents.

This patch implements this in traverse_element: if the display value is not none or contents, we first check whether the contents property should make the element replaced, and if it shouldn't, then we check whether the element itself is replaced or a widget.

Per the spec, an invalid image must be treated as representing a transparent black image with zero natural width and height – in particular, it must not show a broken image icon. We added the method ReplacedContents::zero_sized_invalid_image to implement this.

This patch adds support for image URL references, but not for color gradients, which are treated as invalid images. The reason for this is that currently Servo does not support gradients in ReplacedContentKind. This is left as a follow-up change.

Testing: Some of the existing css/css-content/element-replacement* WPT tests now pass with this patch. We also added some new ones dealing with replacing the document root.

Fixes: #41479
Reviewed in servo/servo#41480

[wpt-pr-bot]

The review process for this patch is being conducted in the Servo project.