style: Make CalcNode the specified representation of <length> and <length-percentage> values.

This is the meat of the patch. There are a couple improvements done in a couple
later patches which should hopefully be straight-forward.

Differential Revision: https://phabricator.services.mozilla.com/D63397

components/style/values/generics/transform.rs

@@ -354,7 +354,7 @@ impl ToAbsoluteLength for SpecifiedLengthPercentage {
         match *self {
             Length(len) => len.to_computed_pixel_length_without_context(),
             Calc(ref calc) => calc.to_computed_pixel_length_without_context(),
-            _ => Err(()),
+            Percentage(..) => Err(()),
         }
     }
 }