Intent to prototype: support percentage value for text-decoration-inset

[Boris Chiou]

Summary:
Per this spec issue, csswg-draft#8403, we'd like to extend the syntax of text-decoration-inset to support <length-percentage> value, instead of <length>. This property is already implemented, but previously only supported length values. Per the resolution of this spec issue, we now intend to support percentage values as well.

The percentage basis depends on box-decoration-break. If we specify box-decoration-break:slice, we have to use the total length of all breaks as the basis. If its value is clone, each fragment is considered individually.

Bug: Bug 2044602

Specification:
https://drafts.csswg.org/css-text-decor-4/#text-decoration-inset-property

Standards Body: CSSWG / W3C

Platform Coverage: All.

Preference: layout.css.text-decoration-inset-percentage.enabled

DevTools Bug: N/A

Extensions Bug: N/A

Use Counter: css_text_decoration_inset

Standards-Positions Discussion:
There is no standards positions discussion because the spec resolution has been merged, and I believe the solution is uncontroversial and the Layout team agrees with this update.

Other Browsers:
- Blink: intent to ship email. 
- WebKit: Not implemented yet. See WebKit Bug 322441 for more details.

web-platform-tests:
parsing: css/css-text-decor/parsing/text-decoration-inset-*.html
rendering: css/css-text-decor/text-decoration-inset-{026|027|028|029|030}.html