Service Worker: Surface Client.id attribute.

LayoutTests/http/tests/serviceworker/resources/client-id-worker.js

+self.onmessage = function(e) {
+  var port = e.data.port;
+  var message = [];
+
+  Promise.resolve()
+    .then(function() {
+        // 1st matchAll()
+        return self.clients.matchAll().then(function(clients) {
+            clients.forEach(function(client) {
+                message.push(client.id);
+              });
+          });
+      })
+    .then(function() {
+        // 2nd matchAll()
+        return self.clients.matchAll().then(function(clients) {
+            clients.forEach(function(client) {
+                message.push(client.id);
+              });
+          });
+      })
+    .then(function() {
+        // Send an array containing ids of clients from 1st and 2nd matchAll()
+        port.postMessage(message);
+      });
+};