Web: implement WaitUntilStrategy (#3739)

src/platform_impl/web/web_sys/worker.js

@@ -0,0 +1,10 @@
+onmessage = event => {
+    const [port, timeout] = event.data
+    const f = () => port.postMessage(undefined)
+
+    if ('scheduler' in this) {
+        scheduler.postTask(f, { delay: timeout })
+    } else {
+        setTimeout(f, timeout)
+    }
+}