Auto merge of #17470 - ferjm:bug1336891.customprops.patch, r=emilio
Auto merge of #17470 - ferjm:bug1336891.customprops.patch, r=emilio

stylo: Implement custom property value getter

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors

https://bugzilla.mozilla.org/show_bug.cgi?id=1336891

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