Fix ability to build release builds

Also fixed a typo in big_red_button script where
we were passing 'gn' to the 'gn' script.

R=abarth@google.com

sky/tools/big_red_button.py

@@ -82,7 +82,7 @@ def main():
 
     run(sky_engine_root, ['git', 'pull', '--rebase'])
     run(sky_engine_root, ['gclient', 'sync'])
-    run(sky_engine_root, ['sky/tools/gn', 'gn', '--android', '--release'])
+    run(sky_engine_root, ['sky/tools/gn', '--android', '--release'])
     # TODO(eseidel): We shouldn't use mojob anymore, it likely will break.
     run(sky_engine_root, ['mojo/tools/mojob.py', 'build', '--android', '--release'])
     # Run tests?

sky/tools/gn

@@ -58,6 +58,7 @@ def to_gn_args(args):
 def main():
   parser = argparse.ArgumentParser(description='A script run` gn gen`.')
   parser.add_argument('--debug', default=True)
+  parser.add_argument('--release', default=False, dest='debug', action='store_false')
   parser.add_argument('--target-os', type=str)
   parser.add_argument('--android', dest='target_os', action='store_const', const='android')
   parser.add_argument('--ios', dest='target_os', action='store_const', const='ios')