If you take a look in the "Recents" tab in the newest Android Phone App you'll find that when you click on a contact, it expands and two buttons appear (Call back and details). Take a look in the picture below:
What are they called? I've browsed through the Material Design element specification, but I can't find anything similar.
These are "flat" buttons. You can take a look at the documentation here: https://www.google.com/design/spec/components/buttons.html