Auto merge of #15083 - potocpav:issue-14497, r=KiChjang

Fix #14497: WeakMediaQueryListVec assumes its contents are still alive

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First time contributor here. Noticed this issue is easily solved, so I did it. I am very unsure about how things work here: Is a test needed? What should it look like? Should the "unwrap" be avoided (making bigger changes to the code)?

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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 #14497  (github issue number if applicable).

- [X] There are tests for these changes

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