Replace deprecated unload handler with pagehide/pageshow in media-control-api.js

[ignat980]

• Chrome has deprecated the unload event; DevTools shows “Unload event listeners are deprecated and will be removed.”
• Replaced window.onunload with a pagehide listener to send unregisterTab.
• Added a pageshow listener to re-register when restoring from the back/forward cache (event.persisted === true).
• This removes the deprecation warning and should keep registration correct across navigations and bfcache restores.