Add toSVGClipPathElement(), and use it

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

As minor clean-up, this cl changes to use toSVGSVGElement() instead of static_cast<SVGSVGElement*>.

BUG=

Review URL: https://chromiumcodereview.appspot.com/23063002

git-svn-id: svn://svn.chromium.org/blink/trunk@156069 bbb929c8-8fbe-4397-9dbb-9b2b20218538