LibWeb+WebContent: Add APIs to control video playback state

This allows for the browser process to control the play/pause state,
whether we paint user agent controls on the video, and whether the video
loops when it finishes playing.

Userland/Services/WebContent/WebContentServer.ipc

@@ -76,4 +76,8 @@ endpoint WebContentServer
     alert_closed() =|
     confirm_closed(bool accepted) =|
     prompt_closed(Optional<String> response) =|
+
+    toggle_video_play_state() =|
+    toggle_video_loop_state() =|
+    toggle_video_controls_state() =|
 }