Introduce toSVGRadialGradientElement(), and use it

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

BUG=N/A

Review URL: https://chromiumcodereview.appspot.com/22897011

git-svn-id: svn://svn.chromium.org/blink/trunk@156441 bbb929c8-8fbe-4397-9dbb-9b2b20218538