style: Fix cursor prefixed aliases to do the right thing.
style: Fix cursor prefixed aliases to do the right thing.

This was an oversight in bug 1520154. We kept the -moz- version in the specified
value but not the computed value.

That's a very peculiar way of making aliases work. This makes them work
consistently as many other aliases instead.

Also, add an assert that would've caught this much much earlier.

Differential Revision: https://phabricator.services.mozilla.com/D40063

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