Upstream service worker `postMessage` tests to WPT

third_party/WebKit/LayoutTests/external/wpt/service-workers/service-worker/resources/postmessage-transferables-worker.js

 var messageHandler = function(port, e) {
     var text_decoder = new TextDecoder;
     port.postMessage({
       content: text_decoder.decode(e.data),
       byteLength: e.data.byteLength
     });
 
     // Send back the array buffer via Client.postMessage.
     port.postMessage(e.data, [e.data.buffer]);
 
     port.postMessage({
       content: text_decoder.decode(e.data),
       byteLength: e.data.byteLength
     });
 };
 
 self.addEventListener('message', e => {
     if (e.ports[0]) {
       // Wait for messages sent via MessagePort.
       e.ports[0].onmessage = messageHandler.bind(null, e.ports[0]);
       return;
     }
     messageHandler(e.source, e);
   });