Focus the sandboxed iframe when chrome://media-app loads.

Currently the host frame is focused, which means keyboard shortcuts
installed on 'window' in the sandboxed iframe will not see events until
the iframe is clicked on or otherwise interacted with.

Bug: b/158707819
Change-Id: I866776c41b90faaa2e2cb7814fc3f714c4e81725
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2239140
Commit-Queue: Zain Afzal <zafzal@google.com>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#777275}

chromeos/components/media_app_ui/resources/js/launch.js

@@ -536,3 +536,11 @@ function installLaunchHandler() {
 }
 
 installLaunchHandler();
+
+// Make sure the guest frame has focus.
+/** @type {!Element} */
+const guest = assertCast(
+    document.querySelector('iframe[src^="chrome-untrusted://media-app"]'));
+guest.addEventListener('load', () => {
+  guest.focus();
+});

chromeos/components/media_app_ui/test/media_app_ui_browsertest.js

@@ -731,6 +731,16 @@ TEST_F('MediaAppUIBrowserTest', 'RelatedFiles', async () => {
   testDone();
 });
 
+// Tests that the guest gets focus automatically on start up.
+TEST_F('MediaAppUIBrowserTest', 'GuestHasFocus', async () => {
+  const guest = document.querySelector('iframe');
+
+  // By the time this tests runs the iframe should already have been loaded.
+  assertEquals(document.activeElement, guest);
+
+  testDone();
+});
+
 // Test cases injected into the guest context.
 // See implementations in media_app_guest_ui_browsertest.js.