LibWeb: Add method for whether tree-counting function is allowed

Some contexts (e.g. descriptors, media conditions) don't allow tree
counting functions, this commit adds an easy way to check if the current
value context is one of those.

Libraries/LibWeb/CSS/Parser/ValueParsing.cpp

@@ -4142,6 +4142,9 @@ RefPtr<CalculatedStyleValue const> Parser::parse_calculated_value(ComponentValue
                 case SpecialContext::TranslateZArgument:
                     // Percentages are disallowed for the Z axis
                     return CalculationContext {};
+                case SpecialContext::DOMMatrixInitString:
+                case SpecialContext::MediaCondition:
+                    return {};
                 }
                 VERIFY_NOT_REACHED();
             });