Auto merge of #29695 - Loirooriol:outline, r=mrobinson

Implement outlines in layout-2020

<!-- Please describe your changes on the following line: -->

This only covers the CSS2 properties: 'outline-width', 'outline-style',
'outline-color', and the shorthand 'outline'.

CSS UI 3 introduced 'outline-offset', which is left for a follow-up.

'outline-color: invert' isn't included either, but note CSS UI says that
conformant UAs may ignore the 'invert' value on platforms that do not
support color inversion of the pixels on the screen.

Tests that are now passing:
- /_mozilla/css/input_insertion_point_empty_a.html
- /_mozilla/css/outlines_simple_a.html
- /_mozilla/css/stacking_context_overflow_relative_outline_a.html
- /_mozilla/mozilla/getPropertyPriority.html
- /css/CSS2/ui/outline-color-001.xht
- /css/CSS2/ui/outline-color-002.xht
- /css/CSS2/ui/outline-color-007.xht
- /css/CSS2/ui/outline-color-008.xht
- /css/CSS2/ui/outline-color-013.xht
- /css/CSS2/ui/outline-color-018.xht
- /css/CSS2/ui/outline-color-023.xht
- /css/CSS2/ui/outline-color-024.xht
- /css/CSS2/ui/outline-color-025.xht
- /css/CSS2/ui/outline-color-031.xht
- /css/CSS2/ui/outline-color-036.xht
- /css/CSS2/ui/outline-color-041.xht
- /css/CSS2/ui/outline-color-046.xht
- /css/CSS2/ui/outline-color-047.xht
- /css/CSS2/ui/outline-color-048.xht
- /css/CSS2/ui/outline-color-049.xht
- /css/CSS2/ui/outline-color-050.xht
- /css/CSS2/ui/outline-color-051.xht
- /css/CSS2/ui/outline-color-052.xht
- /css/CSS2/ui/outline-color-053.xht
- /css/CSS2/ui/outline-color-054.xht
- /css/CSS2/ui/outline-color-058.xht
- /css/CSS2/ui/outline-color-059.xht
- /css/CSS2/ui/outline-color-061.xht
- /css/CSS2/ui/outline-color-062.xht
- /css/CSS2/ui/outline-color-069.xht
- /css/CSS2/ui/outline-color-070.xht
- /css/CSS2/ui/outline-color-071.xht
- /css/CSS2/ui/outline-color-072.xht
- /css/CSS2/ui/outline-color-073.xht
- /css/CSS2/ui/outline-color-074.xht
- /css/CSS2/ui/outline-color-075.xht
- /css/CSS2/ui/outline-color-079.xht
- /css/CSS2/ui/outline-color-081.xht
- /css/CSS2/ui/outline-color-082.xht
- /css/CSS2/ui/outline-color-089.xht
- /css/CSS2/ui/outline-color-090.xht
- /css/CSS2/ui/outline-color-091.xht
- /css/CSS2/ui/outline-color-092.xht
- /css/CSS2/ui/outline-color-093.xht
- /css/CSS2/ui/outline-color-094.xht
- /css/CSS2/ui/outline-color-095.xht
- /css/CSS2/ui/outline-color-099.xht
- /css/CSS2/ui/outline-color-101.xht
- /css/CSS2/ui/outline-color-102.xht
- /css/CSS2/ui/outline-color-109.xht
- /css/CSS2/ui/outline-color-110.xht
- /css/CSS2/ui/outline-color-111.xht
- /css/CSS2/ui/outline-color-112.xht
- /css/CSS2/ui/outline-color-113.xht
- /css/CSS2/ui/outline-color-114.xht
- /css/CSS2/ui/outline-color-115.xht
- /css/CSS2/ui/outline-color-119.xht
- /css/CSS2/ui/outline-color-121.xht
- /css/CSS2/ui/outline-color-122.xht
- /css/CSS2/ui/outline-color-130.xht
- /css/css-ui/outline-001.html
- /css/css-ui/outline-002.html
- /css/css-ui/outline-004.html
- /css/css-ui/outline-007.html
- /css/css-ui/outline-008.html
- /css/css-ui/outline-016.html
- /css/css-ui/outline-018.html
- /css/css-ui/outline-021.html
- /css/css-ui/outline-022.html
- /css/css-ui/outline-color-001.html
- /css/css-ui/outline-style-011.html
- /css/css-ui/outline-style-012.html
- /css/css-ui/outline-style-013.html
- /css/css-ui/outline-style-014.html
- /css/css-ui/parsing/outline-color-computed.html
- /css/css-ui/parsing/outline-color-valid-mandatory.html
- /css/css-ui/parsing/outline-shorthand.html
- /css/css-ui/parsing/outline-style-computed.html
- /css/css-ui/parsing/outline-style-valid.html
- /css/css-ui/parsing/outline-width-valid.html
- /css/css-ui/translucent-outline.html

Also improvements in:
- /_mozilla/mozilla/calc.html
- /css/css-ui/animation/outline-color-interpolation.html
- /css/css-ui/animation/outline-width-interpolation.html
- /css/css-ui/inheritance.html
- /css/css-ui/outline-017.html
- /css/css-ui/parsing/outline-valid-mandatory.html
- /css/css-ui/parsing/outline-width-computed.html
- /css/cssom/cssom-setProperty-shorthand.html
- /css/cssom/getComputedStyle-resolved-colors.html
- /css/cssom/serialize-values.html
- /css/cssom/shorthand-values.html

New failures:
- /css/CSS2/ui/outline-applies-to-005.xht
- /css/CSS2/ui/outline-applies-to-006.xht
- /css/CSS2/ui/outline-color-applies-to-005.xht
- /css/CSS2/ui/outline-color-applies-to-006.xht
- /css/CSS2/ui/outline-style-applies-to-005.xht
- /css/CSS2/ui/outline-style-applies-to-006.xht
- /css/CSS2/ui/outline-width-applies-to-005.xht
- /css/CSS2/ui/outline-width-applies-to-006.xht
  Al of these fail because tables are not implemented yet.
- /css/css-ui/outline-offset.html
  Fails because outline-offset is not implemented yet.
- /css/css-ui/outline-with-padding-001.html
  Fails because the outline doesn't include overflowing contents.
  I don't think this is required by the spec, Firefox fails too.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #29694 (GitHub issue number if applicable)

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