Take WorkerGlobalScope's worker_id out of its Option.

Unsurprisingly, every worker has an id, so there is no need to wrap it in an
Option.

components/script/dom/bindings/global.rs

@@ -125,7 +125,7 @@ impl<'a> GlobalRef<'a> {
     pub fn get_worker_id(&self) -> Option<WorkerId> {
         match *self {
             GlobalRef::Window(_) => None,
-            GlobalRef::Worker(ref worker) => worker.get_worker_id(),
+            GlobalRef::Worker(ref worker) => Some(worker.get_worker_id()),
         }
     }