Add a test for cross process service worker messaging.

LayoutTests/http/tests/serviceworker/resources/postmessage-cross-process-worker.js

+var port;
+var resultport = null;
+var result = null;
+
+self.onmessage = function(e) {
+  var message = e.data;
+  if ('port' in message) {
+    port = message.port;
+    port.onmessage = onMessage;
+  } else if ('done' in message) {
+    result = message.done;
+    if (resultport) {
+      resultport.postMessage(result);
+    }
+  } else if ('resultport' in message) {
+    resultport = message.resultport;
+    if (result) {
+      resultport.postMessage(result);
+    }
+  }
+};
+
+function onMessage(e) {
+  var message = e.data;
+  if ('value' in message) {
+    port.postMessage('Acking value: ' + message.value);
+  } else if ('done' in message) {
+    port.postMessage('quit');
+  }
+}