What this patch do are
- add `onpositionstatechange` event handler on MediaController
- `PositionStateEvent` would be sent to `positionstatechange` event handler
The advantage of doing so is
- to allow us to listen to the position change on the media controller interface (that can be used for testing and the future plan, the media hub)
Differential Revision: https://phabricator.services.mozilla.com/D80791
alwu