The default_app asar was recently changed to reference files inside
`node_modules/` in addition to files inside `default_app/`. The
`js2asar.py` script was updated to interpret what this meant, but the GN
build wasn't. This change somewhat hackily makes the GN build reuse the
`js2asar.py` script so that this and also hopefully any future changes
will work in the GN build as well as the GYP build.
