Auto merge of #11433 - heycam:dummy-base-url, r=bholley
Auto merge of #11433 - heycam:dummy-base-url, r=bholley

Don't re-parse dummy base URL every time in GeckoElement::style_attribute

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 test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

Either:
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because geckolib-only

Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process.

----

r? @bholley

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