Auto merge of #19579 - neerjapancholi:master, r=emilio

Bug 1417725 - (Servo) Add -moz-column-span alias for column-span prop…

…erty.

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This is part of the patch for adding a -moz- alias for column-span. See https://bugzilla.mozilla.org/show_bug.cgi?id=1417725
Clean try run for this patch is at:
https://treeherder.mozilla.org/#/jobs?repo=try&revision=faa9c050c9adf10714953b8c5d5af67683992437&selectedJob=151353901

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