Reland of "Expose events in workers as per latest DOM specification"

LayoutTests/fast/workers/resources/worker-custom-event.js

Index: LayoutTests/fast/workers/resources/worker-custom-event.js
diff --git a/LayoutTests/fast/workers/resources/worker-custom-event.js b/LayoutTests/fast/workers/resources/worker-custom-event.js
new file mode 100644
index 0000000000000000000000000000000000000000..c6a32262b52fa8e2d495029f347add0501c239ba
--- /dev/null
+++ b/LayoutTests/fast/workers/resources/worker-custom-event.js
@@ -0,0 +1,12 @@
+function onmessage(evt)
+{
+    var target = self;
+    target.addEventListener('custom-event', function(e) {
+        postMessage("SUCCESS");
+    }, true);
+
+    var event = new Event('custom-event');
+    target.dispatchEvent(event);
+}
+
+addEventListener("message", onmessage, true);