LibWeb: Plumb calculated StyleValues into CSS::Length

This is a bit hackish, but this way the existance of the calc()
becomes transparent to the user who just wants a Length and doesn't
care where it came from.

Userland/Libraries/LibWeb/CSS/Length.cpp

@@ -76,6 +76,8 @@ const char* Length::unit_name() const
         return "vmax";
     case Type::Vmin:
         return "vmin";
+    case Type::Calculated:
+        return "calculated";
     }
     VERIFY_NOT_REACHED();
 }