Auto merge of #17787 - jdm:invalidvalue, r=SimonSapin

Store COW strings in CSS parser errors when possible.

This should be a straightforward performance improvement in pages with lots of CSS property values that the parser does not understand.

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes do not require tests because there's no functional difference.

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ports/geckolib/error_reporter.rs

@@ -224,7 +224,7 @@ impl<'a> ErrorHelpers<'a> for ContextualParseError<'a> {
             (_, CssParseError::Custom(SelectorParseError::Custom(
                 StyleParseError::PropertyDeclaration(
                     PropertyDeclarationParseError::InvalidValue(property))))) =>
-                ErrorString::Snippet(property.into()),
+                ErrorString::Snippet(property),
 
             (_, CssParseError::Custom(SelectorParseError::UnexpectedIdent(ident))) =>
                 ErrorString::Ident(ident),