Separate disposing of layout data from the GCing of the DOM object reflectors. Change the order of operations when shutting down the script task to ensure that Window globals aren't used after they've been GCed.

components/script/dom/window.rs

@@ -461,6 +461,9 @@ impl<'a, T: Reflectable> ScriptHelpers for JSRef<'a, T> {
 
 impl<'a> WindowHelpers for JSRef<'a, Window> {
     fn clear_js_context(self) {
+        let document = self.Document().root();
+        NodeCast::from_ref(document.r()).teardown();
+
         *self.js_context.borrow_mut() = None;
         *self.browser_context.borrow_mut() = None;
     }