As docs are saying
The symbol files (.sym) can be extracted from the downloaded packages.
As far as I understand from one of this http://nwjs.io/downloads/ or from one provided by the nw-builder. Both have the same structure without any *.sym files. And there is no clue how to extract them. Nor in the docs nor in the www.
There is a link to "Binaries for other platforms" in release notes.
For example at page http://nwjs.io/blog/v0.17.4/ there is a link to https://dl.nwjs.io/v0.17.4/ which contains symbol files we are looking for.