Add LayoutTests to test Blob URL transfer to {Shared,Service}Workers

third_party/WebKit/LayoutTests/external/wpt/workers/support/WorkerFetchURL.js

+onconnect = e => {
+  let port = e.ports[0];
+  port.onmessage = (e) => {
+    fetch(e.data)
+    .then(response => response.text())
+    .then(text => port.postMessage('Worker reply:' + text));
+  };
+}