servo: Merge #20573 - Add support for percentages in column-gap (from Ramshell:issue/20498); r=mbrubeck

<!-- Please describe your changes on the following line: -->
# Changes in this PR
* I've removed the `if product == "gecko"` line in order to make the column_gap type "NonNegativeLengthOrPercentageOrNormal" always.

* Then I've changed this line
```rust
Either::First(len) => len.into(),
```
to
```rust
Either::First(len) => len.0.to_pixel_length(content_inline_size).into(),
```

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach build-geckolib` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #20498  (github issue number if applicable).

<!-- Either: -->
- [X] These changes do not require tests because there are tests already for this in wpt, in particular css/css-align/gaps/column-gap-parsing-001.html for the parsing bits, and css/css-multicol/multicol-gap-percentage-001.html for the layout bits.

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Source-Repo: https://github.com/servo/servo
Source-Revision: ecf71d77cde1ce998b2d2b7fd7a6d45dfe40343e

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 16a3fc0d6220196432027a859c13b1f364217769

servo/components/layout/multicol.rs

@@ -106,9 +106,8 @@ impl Flow for MulticolFlow {
         {
             let column_style = self.block_flow.fragment.style.get_column();
 
-            // FIXME(#20498): This should support percentages too.
             let column_gap = match column_style.column_gap {
-                Either::First(len) => len.into(),
+                Either::First(len) => len.0.to_pixel_length(content_inline_size).into(),
                 Either::Second(_normal) => self.block_flow.fragment.style.get_font().font_size.size(),
             };
 

servo/components/style/properties/longhand/column.mako.rs

@@ -30,17 +30,14 @@ ${helpers.predefined_type(
 )}
 
 
-<%
-# FIXME(#20498): Servo should support percentages in column-gap.
-col_gap_type = "NonNegativeLengthOrPercentageOrNormal" if product == "gecko" else "NonNegativeLengthOrNormal"
-%>
+
 ${helpers.predefined_type(
     "column-gap",
-    "length::%s" % col_gap_type,
+    "length::NonNegativeLengthOrPercentageOrNormal",
     "Either::Second(Normal)",
     extra_prefixes="moz",
     servo_pref="layout.columns.enabled",
-    animation_value_type=col_gap_type,
+    animation_value_type="NonNegativeLengthOrPercentageOrNormal",
     spec="https://drafts.csswg.org/css-multicol/#propdef-column-gap",
     servo_restyle_damage = "reflow",
 )}