Make PushEvent's data payload binary instead of just a string

This exposes binary data as a Body mixin from the Fetch spec,
as discussed on https://github.com/w3c/push-api/issues/83
This shouldn't be a NoInterfaceObject. Per WebIDL:

"The [NoInterfaceObject] extended attribute should not be used on interfaces that are not solely used as supplemental interfaces, unless there are clear Web compatibility reasons for doing so."