Add type annotation to script::dom::document

rustc 1.21.0-nightly (ab40a7cb0 2017-08-13) rejects unless this is present.

components/script/dom/document.rs

@@ -3944,7 +3944,7 @@ impl DocumentMethods for Document {
 }
 
 fn update_with_current_time_ms(marker: &Cell<u64>) {
-    if marker.get() == Default::default() {
+    if marker.get() == u64::default() {
         let time = time::get_time();
         let current_time_ms = time.sec * 1000 + time.nsec as i64 / 1000000;
         marker.set(current_time_ms as u64);