This actually makes sense(?), when running inside an iframe the active
element might be our canvas, while the iframe itself is not active in
the parent window. Since we consume the event, the iframe does not get
focused in Firefox (but does in Chromium-based browsers), so we must
always call focus to handle such occasions.
In some conditions the events might be generated even when the `gamepad`
object is not accessible due to Security Context requirements.
This commit adds a check to avoid firing the handler in those cases.
Luis Sanchez
Fabio Alessandrelli