ideally a silent enabling process.

As many other permissions they must be requested on demand and approved by user each time you need them. This is security related policy. If it was not like this you can spy on user and violate user`s privacy. This is not allowed.

On trusted https domains (like from Google) user can allow usage of some devices automatically after first request and allow their usage each time code request them. But still in this case it will not be silently. Browser will indicate in address bar that code uses camera/microphone etc and user will know this.