style: Properly track whether negative values of calc() are allowed

In order to clamp them at computed value time.

components/style/values/computed/mod.rs

@@ -167,6 +167,7 @@ impl ToComputedValue for specified::CalcLengthOrPercentage {
         self.compute_from_viewport_and_font_size(context.viewport_size(),
                                                  context.style().get_font().clone_font_size(),
                                                  context.style().root_font_size())
+
     }
 }