auto merge of #3666 : ttaubert/servo/issue/3644-privatize-dom, r=Manishearth

This PR removes public fields from all (hope I didn't miss any) DOM structs. Should |Page| be privatized as well? This PR additionally introduces a #[privatize] lint to ensure nobody accidentally re-introduces a public field.

All changesets compile separately if applied in the same order. Hope that helps reviewing but I can of course squash them before merging.

components/script/dom/workernavigator.rs

@@ -13,6 +13,7 @@ use servo_util::str::DOMString;
 
 #[jstraceable]
 #[must_root]
+#[privatize]
 pub struct WorkerNavigator {
     reflector_: Reflector,
 }