[ServiceWorker] Add wpt tests to verify Client.type as 'worker'

third_party/WebKit/LayoutTests/external/wpt/service-workers/service-worker/resources/clients-get-client-types-worker.js

+onmessage = function(e) {
+  if (e.data.cmd == 'GetClientId') {
+    fetch('clientId')
+        .then(function(response) {
+          return response.text();
+        })
+        .then(function(text) {
+          e.data.port.postMessage({clientId: text});
+        });
+  }
+};