Auto merge of #15063 - Wafflespeanut:calc, r=heycam

Cleaning up CalcLengthOrPercentage

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We don't really need enum variants in `CalcLengthOrPercentage`. Given that we already have the information (whether it's `vw`, `ch`, etc.) in the struct fields, we could just store `Option<CSSFloat>` there, and modify our `ToCss` implementation a bit.

cc #15061

r? @Manishearth or anyone interested

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