style: Relax a bit an invalid assertion.

We may end up looking at a non-flushed AuthorStyles object when looking at
whether attribute changes and such may affect style.

Check the styles are clean to preserve the assertion, since if that happens
before the first flush, we may not have updated the quirks_mode field (and
that's fine).

Bug: 1468640
MozReview-Commit-ID: FgVpiTf4qMr

components/style/gecko/wrapper.rs

@@ -178,11 +178,13 @@ impl<'lr> TShadowRoot for GeckoShadowRoot<'lr> {
                 as *const bindings::RawServoAuthorStyles)
         };
 
+
         let author_styles = AuthorStyles::<GeckoStyleSheet>::from_ffi(author_styles);
 
         debug_assert!(
             author_styles.quirks_mode == self.as_node().owner_doc().quirks_mode() ||
-                author_styles.stylesheets.is_empty()
+                author_styles.stylesheets.is_empty() ||
+                author_styles.stylesheets.dirty()
         );
 
         &author_styles.data