style: Better Shadow DOM assertions.

Bug: 1445682
Reviewed-by: xidorn
Differential Revision: https://phabricator.services.mozilla.com/D748

components/style/gecko/wrapper.rs

@@ -171,6 +171,7 @@ impl<'lr> TShadowRoot for GeckoShadowRoot<'lr> {
         let author_styles =
             AuthorStyles::<GeckoStyleSheet>::from_ffi(author_styles);
 
+        debug_assert!(!author_styles.stylesheets.dirty());
         debug_assert!(
             author_styles.quirks_mode == self.as_node().owner_doc().quirks_mode() ||
             author_styles.stylesheets.is_empty()

components/style/stylesheet_set.rs

@@ -263,7 +263,7 @@ where
         Self {
             entries: vec![],
             data_validity: DataValidity::Valid,
-            dirty: true,
+            dirty: false,
         }
     }
 }