We don't need to keep two inline functions because toSVGStopElement(Node*) can support all casting.

BUG=N/A

Review URL: https://chromiumcodereview.appspot.com/23295010

git-svn-id: svn://svn.chromium.org/blink/trunk@156444 bbb929c8-8fbe-4397-9dbb-9b2b20218538