I'm no lawyer and you shouldn't trust me so much on this; however, I felt I should give you at least some hints on how to deal with NW.js application licensing.
First, I quote what is stated on the GitHub Wiki page of the project:
"Since the binary is based on Chromium, multiple open source license notices are needed including the MIT License, the LGPL, the BSD, the Ms-PL and an MPL/GPL/LGPL tri-license."
You can find most of these at http://www.gnu.org/licenses/license-list.en.html.
From my understanding, the MPL/GPL/LGPL tri-license is a license that lets you choose one of the three as a better fit for your project, so you don't need to release your code under an open source license as established by the GPL license, but you can go with a commercial license.
I quote the online documentation of the NW.js project again:
"This doesn't apply to your code and you don't have to open source your code"