style: Add CSS Error for DisallowedImportRule.

- Add new CSS Error
- Add new test case for error
- Ensure that test cases use `replace()` and `replaceSync()`

Differential Revision: https://phabricator.services.mozilla.com/D69423

components/style/stylesheets/rule_parser.rs

@@ -193,7 +193,7 @@ impl<'a, 'i> AtRuleParser<'i> for TopLevelRuleParser<'a> {
                 }
 
                 if let AllowImportRules::No = self.allow_import_rules {
-                    return Err(input.new_custom_error(StyleParseErrorKind::UnexpectedImportRule))
+                    return Err(input.new_custom_error(StyleParseErrorKind::DisallowedImportRule))
                 }
 
                 // FIXME(emilio): We should always be able to have a loader

components/style_traits/lib.rs

@@ -137,6 +137,8 @@ pub enum StyleParseErrorKind<'i> {
     UnexpectedNamespaceRule,
     /// @import must be before any rule but @charset
     UnexpectedImportRule,
+    /// @import rules are disallowed in the parser.
+    DisallowedImportRule,
     /// Unexpected @charset rule encountered.
     UnexpectedCharsetRule,
     /// Unsupported @ rule
@@ -149,7 +151,6 @@ pub enum StyleParseErrorKind<'i> {
     ValueError(ValueParseErrorKind<'i>),
     /// An error was encountered while parsing a selector
     SelectorError(SelectorParseErrorKind<'i>),
-
     /// The property declaration was for an unknown property.
     UnknownProperty(CowRcStr<'i>),
     /// The property declaration was for a disabled experimental property.