Thank you for your very helpful reply.
I haven't used Chrome recently but, on reflection, it could be that pages in Facebook have utilised it.
As for whether any pages are open in Chrome, I can't be sure, but I have nothing open at the moment.
A matter of concern is that I haven't noticed this entry on the 'battery' page until recently, and I don't think I have consciously made any changes to settings. The only thing I did do recently was to uninstall Google+.