style: Simplify calc expressions earlier.

This simplifies a bit the code, and guarantees that all calc()s have percentages
and lengths.

I also wanted to remove unclamped_length() / specified_percentage() (for the
same reason as the above patch), but they're needed for animations for now. When
I implement min() / max() for <length-percentage> they'll be fixed.

Differential Revision: https://phabricator.services.mozilla.com/D60194

components/style/values/generics/transform.rs

@@ -377,7 +377,7 @@ impl ToAbsoluteLength for ComputedLengthPercentage {
             // distance without any layout info.
             //
             // FIXME(emilio): This looks wrong.
-            None => Ok(self.length_component().px()),
+            None => Ok(self.resolve(Zero::zero()).px()),
         }
     }
 }