Worker: Import "imported/wpt/workers" tests (Retry)

third_party/WebKit/LayoutTests/imported/wpt/workers/support/ErrorEvent.js

+onmessage = function(evt)
+{
+    throw(new Error(evt.data));
+}
+
+onerror = function(message, location, line, col)
+{
+    postMessage( {"message": message, "filename": location, "lineno": line, "colno": col} );
+    return false; // "not handled" so the error propagates up to the Worker object
+}