Fixed panic due to GC while parsing
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Don't trace the incomplete parsing contexts during GC, as parsing triggers GC while holding a mutable borrow of the context.
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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#21020
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- [X] These changes do not require tests because fixing a panic
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This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/21028)
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527d874bc1
