build: [gn] include node_modules in default_app asar

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.

build/run-in-dir.py

@@ -0,0 +1,11 @@
+import sys
+import os
+import subprocess
+
+def main(argv):
+  cwd = argv[1]
+  os.chdir(cwd)
+  os.execv(sys.executable, [sys.executable] + argv[2:])
+
+if __name__ == '__main__':
+  main(sys.argv)