Introduce toSVGGradientElement(), use it

As a step to change static_cast with toSVGXXX, static_cast<SVGGradientElement*> can
be changed with toSVGGradientElement().

BUG=

Review URL: https://chromiumcodereview.appspot.com/19729002

git-svn-id: svn://svn.chromium.org/blink/trunk@154610 bbb929c8-8fbe-4397-9dbb-9b2b20218538