For a little bit I was using a Nexus 10 as a secondary display - I used Google Chrome's remote desktop to view a separate X session, with Synergy for sharing my input devices between them (which is now paid for?!). Worked great, except you couldn't see the mouse cursor on the Nexus... :( Ultimately it was more annoying than helpful. (Edit: just realised that's broadcasting in the opposite direction to what you're asking for)

http://web.airdroid.com/ maybe? Lots of drawbacks, though, and not 1:1 mirroring I believe, so may not fit your needs. But seems to support whatsapp via screen mirroring + custom keyboard (probably?). Haven't tested since I'm currently on iOS.

Koush's Mirror app[1] works alright with his Allcast Receiver Chrome app. The interesting thing is that using it actually revealed a (broken) second screen mode in one of the apps I use, didn't know Android had APIs for that. Perhaps it's intended for Chromecast/Miracast and whatever Mirror does looks like that.

[1] https://play.google.com/store/apps/details?id=com.koushikdut...