Upstream service worker `postMessage` tests to WPT

third_party/WebKit/LayoutTests/http/tests/serviceworker/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);
-  });