Auto merge of #16607 - canaltinova:moz-transform-var, r=SimonSapin
Auto merge of #16607 - canaltinova:moz-transform-var, r=SimonSapin

Fix variable serialization bug on shorthand alias properties

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-moz-transform were having a problem with variable serialization. It wasn't printing transform's value and transform wasn't printing -moz-transform's if their value was a variable. This PR fixes that.
Reduces stylo -moz-transform failures to 2: https://treeherder.mozilla.org/#/jobs?repo=try&revision=3fd3a57873e5e98dfcb173eef4b13822a66fdf6a

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