Use toSVGPathElement() instead of static_cast<>

Though there is toSVGPathElement(), some files still use static_cast<SVGPathElement*>.
To remove all static_cast<> use, we need to change argument from SVGElement to Element.

BUG=

Review URL: https://chromiumcodereview.appspot.com/19914002

git-svn-id: svn://svn.chromium.org/blink/trunk@154621 bbb929c8-8fbe-4397-9dbb-9b2b20218538