stylo: Expose -moz-system-metric and -moz-empty-except-children-with-localname.
stylo: Expose -moz-system-metric and -moz-empty-except-children-with-localname.

They don't have the flag in Gecko.

Though I wonder if we could unship them from non-UA sheets.

Bug: 1396048
Reviewed-by: bholley
MozReview-Commit-ID: LGzGDjCZpJC

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