Support 'paint-order' from SVG2.

R=pdr@chromium.org
BUG=223766

Review URL: https://codereview.chromium.org/14907011

Patch from Erik Dahlström <ed@opera.com>.

git-svn-id: svn://svn.chromium.org/blink/trunk@156571 bbb929c8-8fbe-4397-9dbb-9b2b20218538