style: Separate the XBL and shadow dom styling bits.

Bug: 1441022
Reviewed-by: xidorn
MozReview-Commit-ID: 2W0BmZ8wWXg

components/style/stylesheet_set.rs

@@ -263,7 +263,7 @@ where
         Self {
             entries: vec![],
             data_validity: DataValidity::Valid,
-            dirty: false,
+            dirty: true,
         }
     }
 }
@@ -597,6 +597,11 @@ where
         self.collection.dirty
     }
 
+    /// Whether the collection is empty.
+    pub fn is_empty(&self) -> bool {
+        self.collection.len() == 0
+    }
+
     fn collection_for(
         &mut self,
         _sheet: &S,