<!-- 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
f944fec6d3