Factor SVG root painter code into SVGRootPainter

This patch moves the RenderSVGRoot paint code into SVGRootPainter. This
is a pure code move (no real changes).

BUG=412088

Review URL: https://codereview.chromium.org/666163005

git-svn-id: svn://svn.chromium.org/blink/trunk@184125 bbb929c8-8fbe-4397-9dbb-9b2b20218538