Bug 1655204 - part2 : add a testcase to ensure 'stop' would stop controlling media. r=chunmin

Differential Revision: https://phabricator.services.mozilla.com/D84876

dom/media/mediacontrol/tests/browser_media_control_keys_event.js

@@ -31,6 +31,11 @@ add_task(async function testPlayPauseAndStop() {
   await generateMediaControlKey("stop");
   await checkOrWaitUntilMediaStoppedPlaying(tab, testVideoId);
 
+  info(`we have stop controlling media, pressing 'play' won't resume media`);
+  // Not expect playback state change, so using ChromeUtils's method directly.
+  ChromeUtils.generateMediaControlKey("play");
+  await checkOrWaitUntilMediaStoppedPlaying(tab, testVideoId);
+
   info(`remove tab`);
   await BrowserTestUtils.removeTab(tab);
 });