Drop RenderSVGResource::{fill,stroke}PaintingResource shims

Makes for slightly less branchy code in some places, otherwise very little
difference.

Review URL: https://codereview.chromium.org/577583002

git-svn-id: svn://svn.chromium.org/blink/trunk@182058 bbb929c8-8fbe-4397-9dbb-9b2b20218538