Add toSVGTextPathElement(), and use it

As a step to clean-up static_cast<SVGXXX>, static_cast<SVGTextPathElement*> can
be changed with toSVGTextPathElement().

BUG=

Review URL: https://chromiumcodereview.appspot.com/23023002

git-svn-id: svn://svn.chromium.org/blink/trunk@156060 bbb929c8-8fbe-4397-9dbb-9b2b20218538