Auto merge of #14421 - upsuper:patch-2, r=emilio

Treat top-level number in calc() invalid

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This should probably considered as a temporary fix (for [bug 1321206](https://bugzilla.mozilla.org/show_bug.cgi?id=1321206)), to avoid assertion when trying to serialize calc value with only numbers. Certain properties (e.g. `line-height`) would eventually need to keep numbers inside calc.

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Currently, CalcLengthOrPercentage doesn't actually keep the number value. If we don't treat it invalid, we can end up generating empty `calc()` value when one contains numbers (e.g. `calc(1)`), which would violate assertion elsewhere that `calc` must not be empty.

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