This bug tracks the effort to delete BrowserMediaPlayerManager.

It includes:
- Potentially moving RemoteMediaPlayerManager from src/chrome to src/content (would still need to be able to create a RemoteMediaPlayerBridge from src/content).
- Removing all unused code from BrowserMediaPlayerManager (including fullscreen related logic).
- Merging code from BrowserMediaPlayerManager into RemoteMediaPlayerManager, and potentially deleting BMPM.