Introduce toSVGPolygonElement(), and use it

As a step to clean-up static_cast<SVGXXX>, toSVGPolygonElement() is added to clean-up
static_cast<SVGPolygonElement*>.

BUG=

Review URL: https://chromiumcodereview.appspot.com/23566016

git-svn-id: svn://svn.chromium.org/blink/trunk@156912 bbb929c8-8fbe-4397-9dbb-9b2b20218538