[Telemetry SWX] introduce iframeReady promise

iframeReady promise is needed to be sure that iframe is ready for
incoming messages. Otherwise postMessage to the iframe will be failed.

Bug: b:158566609

Change-Id: I65cdb7fb23bdda49b931500ab12d45eddba2ca65
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2424318Reviewed-by: Mahmoud Gawad <mgawad@google.com>
Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Commit-Queue: Oleh Lamzin <lamzin@google.com>
Cr-Commit-Position: refs/heads/master@{#811963}

chromeos/components/telemetry_extension_ui/resources/trusted.js

@@ -944,3 +944,15 @@ untrustedMessagePipe.registerHandler(
 untrustedMessagePipe.registerHandler(
     dpsl_internal.Message.PROBE_TELEMETRY_INFO,
     (message) => telemetryProxy.handleProbeTelemetryInfo(message));
+
+/**
+ * Promise that resolves once the iframe is ready to receive messages.
+ * @type {!Promise<undefined>}
+ */
+const iframeReady = new Promise(resolve => {
+  // Wait for 'load' (and not DOMContentLoaded) to ensure the subframe has been
+  // loaded and is ready to respond to postMessage.
+  window.addEventListener('load', () => {
+    resolve();
+  });
+});