Refactor ServiceWorker tests for inclusion in WPT

third_party/WebKit/LayoutTests/http/tests/serviceworker/navigation-preload/resources/get-state-worker.js

-// This worker listens for commands from the page and messages back
-// the result.
-
-function handle(message) {
-  const np = self.registration.navigationPreload;
-  switch (message) {
-    case 'getState':
-      return np.getState();
-    case 'enable':
-      return np.enable();
-    case 'disable':
-      return np.disable();
-    case 'setHeaderValue':
-      return np.setHeaderValue('insightful');
-  }
-  return Promise.reject('bad message');
-}
-
-self.addEventListener('message', e => {
-    e.waitUntil(handle(e.data).then(result => e.source.postMessage(result)));
-  });