[w3c/gamepad] GamepadHapticActuator needs to define the situation of loss focus (#58)

In the spec of [pulse](https://w3c.github.io/gamepad/extensions.html#dom-gamepadhapticactuator-pulse()), we have already defined how to submit a duration for a pulse vibration. But, we didn't think about when we submit a very long pulse vibration, then we leave our focus to another tab. We should let the controller keep vibrating? Or we should stop it when we loss focus.

@toji , I think you might have some thought about this.

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