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

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

…erty.

<!-- Please describe your changes on the following line: -->
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

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/19579)
<!-- Reviewable:end -->

components/style/properties/longhand/column.mako.rs

@@ -63,7 +63,8 @@ ${helpers.predefined_type(
 ${helpers.single_keyword("column-span", "none all",
                          products="gecko", animation_value_type="discrete",
                          gecko_pref="layout.css.column-span.enabled",
-                         spec="https://drafts.csswg.org/css-multicol/#propdef-column-span")}
+                         spec="https://drafts.csswg.org/css-multicol/#propdef-column-span",
+                         extra_prefixes="moz")}
 
 ${helpers.single_keyword("column-rule-style",
                          "none hidden dotted dashed solid double groove ridge inset outset",