Auto merge of #18549 - nnethercote:fix-enclosing-crash, r=heycam

Fix a panic in Stylo memory reporting.

`MallocSizeOfOps::enclosing_size_of_op` is an `Option<>` type, and the panic in
question is caused by not providing a value in a case where it's needed for
measuring a HashSet.

HashMaps and HashSets are common enough that it makes sense to make
`enclosing_size_of_op` non-optional, which this patch does.

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