Implement outlines in layout-2020

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.

components/style/properties/longhands/outline.mako.rs

@@ -14,7 +14,7 @@ ${helpers.predefined_type(
     "outline-color",
     "Color",
     "computed_value::T::currentcolor()",
-    engines="gecko servo-2013",
+    engines="gecko servo-2013 servo-2020",
     initial_specified_value="specified::Color::currentcolor()",
     animation_value_type="AnimatedColor",
     ignored_when_colors_disabled=True,
@@ -26,7 +26,6 @@ ${helpers.predefined_type(
     "OutlineStyle",
     "computed::OutlineStyle::none()",
     engines="gecko servo-2013 servo-2020",
-    servo_2020_pref="layout.2020.unimplemented",
     initial_specified_value="specified::OutlineStyle::none()",
     animation_value_type="discrete",
     spec="https://drafts.csswg.org/css-ui/#propdef-outline-style",
@@ -37,7 +36,6 @@ ${helpers.predefined_type(
     "BorderSideWidth",
     "crate::values::computed::NonNegativeLength::new(3.)",
     engines="gecko servo-2013 servo-2020",
-    servo_2020_pref="layout.2020.unimplemented",
     initial_specified_value="specified::BorderSideWidth::Medium",
     computed_type="crate::values::computed::NonNegativeLength",
     animation_value_type="NonNegativeLength",

components/style/properties/shorthands/outline.mako.rs

@@ -5,7 +5,7 @@
 <%namespace name="helpers" file="/helpers.mako.rs" />
 
 <%helpers:shorthand name="outline"
-                    engines="gecko servo-2013"
+                    engines="gecko servo-2013 servo-2020"
                     sub_properties="outline-color outline-style outline-width"
                     derive_serialize="True"
                     spec="https://drafts.csswg.org/css-ui/#propdef-outline">