Generate HTMLElementFactory with Python

This CL replaces the old Perl-based HTMLElementFactory with
a Python implementation based in jinja2 templates.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/27009005

git-svn-id: svn://svn.chromium.org/blink/trunk@159702 bbb929c8-8fbe-4397-9dbb-9b2b20218538