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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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors

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- [x] These changes do not require tests because it's a refactor

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tests/wpt/mozilla/tests/mozilla/calc.html

@@ -40,13 +40,13 @@ var widthTests = [
 
     // Alphabetical order
     ['calc(0ch + 0px + 0pt + 0pc + 0in + 0cm + 0mm + 0rem + 0em + 0ex + 0% + 0vw + 0vh + 0vmin + 0vmax)',
-     'calc(0ch + 0em + 0ex + 0px + 0rem + 0vh + 0vmax + 0vmin + 0vw + 0%)',
+     'calc(0ch + 0em + 0ex + 0rem + 0vh + 0vmax + 0vmin + 0vw + 0px + 0%)',
      '0px'],
 
     // Simplification
     ['calc((2 - 1) * 10px)', '10px', '10px'],
     ['calc(((3 - 1) * (8 + 4)) * 10px)', '240px', '240px'],
-    ['calc(5 * (20px / 2 + 7 * (3em + 12px/4 + (8 - 2) * 2rem)))', 'calc(105em + 155px + 420rem)', '8555px'],
+    ['calc(5 * (20px / 2 + 7 * (3em + 12px/4 + (8 - 2) * 2rem)))', 'calc(105em + 420rem + 155px)', '8555px'],
 
 ];