Auto merge of #20430 - aeweston98:aew-issue7084, r=jdm

Pass new method in CollectServoSizes to get size of ObjectPrivateVisitor

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These changes are the servo changes to allow the measurement of DOM objects with the ObjectPrivateVisitor API. Two new functions have been added MemoryReporter and get_size, which are passed as function pointers to two updated mozjs functions. Currently these changes rely on a branch version of mozjs, which has an [open pull request](https://github.com/servo/rust-mozjs/pull/409).

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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 #7084 (github issue number if applicable).

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- [ ] There are tests for these changes OR
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