You can't, there is no System API to access that history of songs.
That being said, there is one way you can capture the list for future - you will need access to user's notifications. Each time a notification for "Now Playing" occurs, you can capture that data and make your own list within the app.
You can use NotificationListenerService to determine if a new notification is for "Now Playing" feature or not. If it is then you can do something with that data.
There is already an app out there that uses this functionality - https://play.google.com/store/apps/details?id=com.greatbytes.nowplayinghistory.