Chrome prevents Macbook from sleeping
I had the same issue where Chrome was preventing sleep, i.e., Activity Monitor Energy tab showed that. And I discovered that when I would close a tab running "https://web.whatsapp.com/", that the "Preventing Sleep" column would go to No. When I would open that web site again, it would go back to "Yes".

I found this blog discussing it:
http://webdiary.com/2015/01/29/whatsappweb/

That correlates with the output of running pmset -g assertions, which shows:

   pid 229(coreaudiod): [0x00066e6300010845] 01:19:47 PreventUserIdleSystemSleep named: "com.apple.audio.e8-07-bf-12-d6-97:output.context.preventuseridlesleep" 
    Created for PID: 2748.