Scheduling an selectionchange event does not set "has scheduled selectionchange event" to true

https://w3c.github.io/selection-api/#scheduling-selectionhange-event
https://w3c.github.io/selection-api/#dfn-has-scheduled-selectionchange-event

The "has scheduled selectionchange event" boolean is never set to true in the spec. This is probably a missing step in [section 6.2.1](https://w3c.github.io/selection-api/#scheduling-selectionhange-event).