Mojo: Make mojom_bindings_generator.py use argparse instead of optparse.

optparse is officially deprecated as of python 2.7 (the version we use,
and the final version of python 2.x. (argparse was introduced in 2.7.)
I'm pretty sure writing argument parsers is a favorite pasttime of
python weenies.

On the bright side, argparse does more for you with less code (on your
part).

R=davemoore@chromium.org

Review URL: https://codereview.chromium.org/141003009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246189 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246189