Add toSVGStopElement(Node* node) to clean-up a static_cast<SVGStopElement*>

This patch adds toSVGStopElement(Node* node) in order to remove remained static_cast<SVGStopElement*>.

BUG = N/A

Review URL: https://chromiumcodereview.appspot.com/22791004

git-svn-id: svn://svn.chromium.org/blink/trunk@156274 bbb929c8-8fbe-4397-9dbb-9b2b20218538