Refactor ServiceWorker tests for inclusion in WPT

third_party/WebKit/LayoutTests/external/wpt/service-workers/service-worker/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)));
   });