style: Make XBL / Shadow DOM use something more light-weight than a Stylist.

It's just a struct aggregating stylesheets + CascadeData, with a quirks_mode
parameter because XBL sucks so bad.

Bug: 1436059
Reviewed-by: xidorn
MozReview-Commit-ID: 7q99tSNXo0K

components/style/stylesheet_set.rs

@@ -583,6 +583,20 @@ impl<S> AuthorStylesheetSet<S>
 where
     S: StylesheetInDocument + PartialEq + 'static,
 {
+    /// Create a new empty AuthorStylesheetSet.
+    #[inline]
+    pub fn new() -> Self {
+        Self {
+            collection: Default::default(),
+            invalidations: StylesheetInvalidationSet::new(),
+        }
+    }
+
+    /// Whether anything has changed since the last time this was flushed.
+    pub fn dirty(&self) -> bool {
+        self.collection.dirty
+    }
+
     fn collection_for(
         &mut self,
         _sheet: &S,
@@ -593,6 +607,17 @@ where
 
     sheet_set_methods!("AuthorStylesheetSet");
 
+    /// Iterate over the list of stylesheets.
+    pub fn iter(&self) -> StylesheetCollectionIterator<S> {
+        self.collection.iter()
+    }
+
+    /// Mark the sheet set dirty, as appropriate.
+    pub fn force_dirty(&mut self) {
+        self.invalidations.invalidate_fully();
+        self.collection.set_data_validity_at_least(DataValidity::FullyInvalid);
+    }
+
     /// Flush the stylesheets for this author set.
     ///
     /// `host` is the root of the affected subtree, like the shadow host, for