make_event_factory.py should use a template

We've gotten way smarter about generating CPP code from Python. This CL
brings make_event_factory.py up to snuff with our latest Jinja2 technology.

Also, teach check-webkit-style that we have Python code in Source/build/scripts
to stop triggering spurious pylint failures.

R=adamk@chromium.org

Review URL: https://codereview.chromium.org/60363003

git-svn-id: svn://svn.chromium.org/blink/trunk@161377 bbb929c8-8fbe-4397-9dbb-9b2b20218538