Factor SVG root inline box paint code into SVGRootInlineBoxPainter

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

BUG=412088

Review URL: https://codereview.chromium.org/669733011

git-svn-id: svn://svn.chromium.org/blink/trunk@184133 bbb929c8-8fbe-4397-9dbb-9b2b20218538