[gamepad] Polling access point

Hi,

It looks like the access point for the polling part of the API
(navigator.gamepads[]) is not a good idea.

Based on a prototype implementation, pages seem to have a tendency to
enumerate Navigator. When the .gamepads[] attribute is accessed, it causes
possibly expensive background resources to be created to access hardware,
even though the content is not really interested in reading data from the
hardware.

Possible solutions:
- change from navigator.gamepads[] to navigator.gamepads() (or
navigator.getGamepads()) to be more explicit about when the API is actually
being used
- require something to activate the API (meta tag, calling some sort of
"start" function)
- require registering for at least one gamepad-related event before data is
provided in gamepads[].
- make .gamepads[] non-enumerable

Any thoughts or other suggestions?

Thanks,
scott