It turns out that the new icon is actually something more strange than we first thought: its an update to the stock Android SMS app. The app used to be called Messaging, and has now been rebranded as Messenger. This is strange because Google has been putting almost no effort into the stock Android apps for a while, and has been focusing more on its suite of Google apps. A Google spokesperson said that it is a matter of choice:
The spokesperson also confirmed that Hangouts will be included on the Nexus 6 in addition to the Messenger app. This isn't a surprise because the stock Email app is always included along with the Gmail app. Still, it's good to know what's going on.
source: Android Police