style: Allow placeholder import sheets.

This is necessary because we can't create GeckoStyleSheets off-main-thread,
so we need a placeholder until it can be filled in.

Bug: 1454030
Reviewed-by: emilio
MozReview-Commit-ID: ssRme4fLYg

components/style/stylesheet_set.rs

@@ -478,7 +478,7 @@ where
         sheet: &S,
         guard: &SharedRwLockReadGuard,
     ) -> &mut SheetCollection<S> {
-        let origin = sheet.contents(guard).origin;
+        let origin = sheet.origin(guard);
         self.collections.borrow_mut_for_origin(&origin)
     }