[SVG] SVGRenderStyle::diff() doesn't handle paint-order.

Paint order changes should cause a StyleDifferenceRepaint - otherwise
we're not triggering resource repaints.

R=pdr@chromium.org,schenney@chromium.org,ed@opera.com

Review URL: https://codereview.chromium.org/60183006

git-svn-id: svn://svn.chromium.org/blink/trunk@161560 bbb929c8-8fbe-4397-9dbb-9b2b20218538