Record DevTools metadata events on worker threads when starting recoring tracing based Timeline

LayoutTests/inspector/timeline/tracing/resources/worker.js

Index: LayoutTests/inspector/timeline/tracing/resources/worker.js
diff --git a/LayoutTests/inspector/timeline/tracing/resources/worker.js b/LayoutTests/inspector/timeline/tracing/resources/worker.js
new file mode 100644
index 0000000000000000000000000000000000000000..59330498eb1529e5853cc031a7d3e1a5bb4ac297
--- /dev/null
+++ b/LayoutTests/inspector/timeline/tracing/resources/worker.js
@@ -0,0 +1,9 @@
+onmessage = function(event) {
+    doWork();
+    setInterval(doWork, 0);
+};
+var message_id = 0;
+function doWork()
+{
+    postMessage("Message #" + message_id++);
+}